grk7a (also called GRK7-1) is one of two zebrafish co-orthologs of vertebrate GRK7, the cone opsin kinase; its paralog grk7b arose in the teleost-specific genome duplication. It is a G protein-coupled receptor kinase of the AGC serine/threonine kinase family, with an N-terminal RGS-homology domain, a central protein kinase domain and a C-terminal CAAX motif for geranylgeranyl membrane anchoring. grk7a is expressed in all cone photoreceptor subtypes and is the predominant Grk7 protein of the larval retina; it is localized to cone outer segments. Recombinant Grk7a phosphorylates light-activated rhodopsin with a Vmax far higher than that of the rod kinase Grk1a or of its paralog Grk7b. By phosphorylating light-activated cone opsins it initiates their shut-off, and loss of grk7a (morpholino knockdown or knockout) slows recovery of the cone photoresponse and lowers temporal contrast sensitivity; cone Grk1b shares this role, so knockout effects are modest. Grk7a is phosphorylated in a cAMP/PKA-dependent manner in the dark, and it mediates the slowing of cone recovery caused by raised cAMP. grk7a is also expressed in the pineal organ, where it is the main kinase that inactivates the photoproduct of the bistable opsin parapinopsin.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004672 protein kinase activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro-based protein kinase activity. Correct but far less informative than the demonstrated rhodopsin kinase activity of Grk7a. Reason: grk7a has a complete protein kinase domain and its recombinant product phosphorylates light-activated rhodopsin (PMID:16787417). The specific activity, rhodopsin kinase activity (GO:0050254), is a descendant of this term and is already annotated by IDA and IBA. The same action was taken for the grk7b paralog. Proposed replacements: rhodopsin kinase activity Supporting Evidence: PMID:16787417 The recombinant GRKs phosphorylated light-activated rhodopsin |
| GO:0004674 protein serine/threonine kinase activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro-based serine/threonine kinase activity. True (GRKs phosphorylate Ser/Thr on activated receptors) but less specific than rhodopsin kinase activity. Reason: Rhodopsin kinase activity (GO:0050254) is the specific, experimentally supported activity (PMID:16787417); UniProt records both the L-seryl and L-threonyl [rhodopsin] reactions for this entry. Replace with the specific term, as done for grk7b. Proposed replacements: rhodopsin kinase activity Supporting Evidence: PMID:16787417 The recombinant GRKs phosphorylated light-activated rhodopsin |
| GO:0004703 G protein-coupled receptor kinase activity | IEA GO_REF:0000120 | MODIFY | Summary: GPCR kinase activity from ARBA/InterPro. Correct at the family level; the opsins are GPCRs. Rhodopsin kinase activity is its more specific child. Reason: The receptor substrate is known: light-activated opsin (PMID:16787417). The specific child term GO:0050254 is preferred and is already present by IDA/IBA. Consistent with the grk7b review. Proposed replacements: rhodopsin kinase activity Supporting Evidence: PMID:16787417 The recombinant GRKs phosphorylated light-activated rhodopsin |
| GO:0005524 ATP binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: ATP binding inferred from the kinase domain (ATP-binding motif residues 192-200 and K215 in UniProt). Reason: ATP is the phosphate donor in the rhodopsin kinase reaction, so binding is certainly correct, but it is a mechanistic sub-function of the kinase activity rather than an informative function in its own right. Supporting Evidence: file:DANRE/grk7a/grk7a-uniprot.txt Reaction=L-seryl-[rhodopsin] + ATP = O-phospho-L-seryl-[rhodopsin] + |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Family-level IBA (GRK node including Drosophila Gprk2 and human GRK4). GRKs are cytosolic kinases that are recruited to activated receptors at membranes. Reason: Not wrong for a soluble-domain kinase, but Grk7a is geranylgeranylated (C-terminal CAAX, ...CTLL) and has been localized to cone outer segments, the membrane-rich compartment that holds the opsin substrate (PMID:16787417). The outer segment and membrane are the informative locations; cytoplasm is retained as non-core, matching the grk7b review. Supporting Evidence: PMID:16787417 In situ hybridization and immunohistochemical studies localized both GRK1B and GRK7-1 in the cone outer segments |
| GO:0007165 signal transduction | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Very broad InterPro-derived process term. Reason: grk7a acts in phototransduction (shut-off of activated cone opsin), which is a kind of signal transduction, but the informative processes are photoresponse recovery and phototransduction, which are annotated separately with experimental evidence. Supporting Evidence: PMID:30372740 Our results suggest Grk1b and Grk7a both contribute to signal termination in cones |
| GO:0007601 visual perception | IMP PMID:21299656 Cone arrestin confers cone vision of high temporal resolutio... | KEEP AS NON CORE | Summary: Renninger et al. 2011 compared cone arrestin (arr3a) morphants with Grk7a morphants (ZDB-MRPHLNO-050824-1) using ERG and optokinetic behavior. Grk7a acts in initial cone response recovery, and loss of grk7a reduces temporal contrast sensitivity in the OKR. Reason: The behavioral OKR deficit is a downstream, organism-level consequence of slower cone recovery. It is real (also seen with the TALEN knockout, PMID:30372740, though smaller), but visual perception is not a process that the kinase carries out; the core process is photoresponse recovery. Abstract-only reference; deferring to the ZFIN curator on the assay. Supporting Evidence: PMID:21299656 Arr3a activity partially overlapped with the function of the cone-specific kinase Grk7a involved in initial response recovery. PMID:30372740 A very small but statistically significant decrease was observed for both grk7a−/− and grk1b−/− larvae at 6 dpf compared to wildtype (Fig. 6A). |
| GO:0007603 phototransduction, visible light | IMP PMID:16039565 Knockdown of cone-specific kinase GRK7 in larval zebrafish l... | ACCEPT | Summary: Morpholino knockdown of grk7a delayed cone photoresponse recovery in ERG and reduced temporal contrast sensitivity, especially in bright light (Rinner et al. 2005). Reason: Phosphorylation of the light-activated opsin is the first step of the shut-off branch of the phototransduction cascade, so Grk7a participates directly in phototransduction in cones. The knockout (PMID:30372740) confirms a smaller but significant effect. The more specific process, photoresponse recovery, is also annotated. Supporting Evidence: PMID:16039565 Photoresponse recovery in Grk7a-deficient larvae was delayed in electroretinographic measurements, and temporal contrast sensitivity was reduced, particularly under bright-light conditions. |
| GO:0009881 photoreceptor activity | IMP PMID:16039565 Knockdown of cone-specific kinase GRK7 in larval zebrafish l... | REMOVE | Summary: "Photoreceptor activity" is the molecular function of absorbing and responding to light (the activity of opsins). Grk7a is a kinase that inactivates the light-activated photoreceptor protein; it does not absorb light. Reason: The term definition ("absorbing and responding to incidental electromagnetic radiation") describes a chromophore-bearing photoreceptor protein, not a kinase acting on it. A knockdown phenotype (delayed photoresponse recovery) cannot establish a molecular function, and the paper's own data are about cone response recovery. The real activity of grk7a is rhodopsin kinase activity (GO:0050254), already annotated by IDA (PMID:16787417), and the phenotype is correctly captured by the phototransduction and photoresponse recovery annotations. This is a mistyped MF, not a disputed finding. Supporting Evidence: PMID:16039565 These results show that function of a cone-specific kinase is essential for cone vision in the zebrafish retina PMID:16787417 The recombinant GRKs phosphorylated light-activated rhodopsin |
| GO:0009966 regulation of signal transduction | IBA GO_REF:0000033 | ACCEPT | Summary: Family-level IBA for GRKs: receptor phosphorylation desensitizes GPCR signalling. Reason: Grk7a limits opsin signalling by phosphorylating the activated receptor, which is regulation (termination) of signal transduction. The IBA node is appropriate for the whole GRK family and grk7a lies within it; there is no evidence of loss of the function. The more specific experimental process terms (photoresponse recovery) are annotated separately. The same action was taken for grk7b. Supporting Evidence: PMID:30372740 Our results suggest Grk1b and Grk7a both contribute to signal termination in cones |
| GO:0016020 membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Membrane location from UniProt subcellular location (lipid anchor, by similarity to bovine GRK7). Reason: Grk7a carries a C-terminal CAAX (...CTLL) geranylgeranylation motif, and GRK7s are anchored by a geranylgeranyl moiety (PMID:29321241); its substrate is the membrane opsin of the cone outer segment (PMID:16787417). A more specific location is proposed below (cone photoreceptor outer segment). Supporting Evidence: PMID:29321241 This indicates that, whereas the GRK1As are anchored by a farensyl moiety, the GRK1Bs and the GRK7s are anchored by a geranylgeranyl moiety. |
| GO:0016020 membrane | ISS GO_REF:0000024 | ACCEPT | Summary: Membrane location transferred from bovine GRK7 (Q8WMV0). Reason: Same reasoning as the IEA row: geranylgeranyl anchoring conserved in all GRK7s, and outer-segment localization shown for zebrafish GRK7-1 (PMID:16787417). Supporting Evidence: PMID:29321241 This indicates that, whereas the GRK1As are anchored by a farensyl moiety, the GRK1Bs and the GRK7s are anchored by a geranylgeranyl moiety. |
| GO:0036368 photoresponse recovery | IMP PMID:30372740 Grk1b and Grk7a Both Contribute to the Recovery of the Isola... | ACCEPT | Summary: TALEN knockout of grk7a (exon-1 7-bp deletion/6-bp insertion, truncation at 53 aa, no detectable Grk7 protein) slows recovery of the isolated cone mass receptor potential in paired-flash ERG at 5 dpf (recovery half-life 3.0 s vs 1.7 s in wild type). Reason: This is the core process of grk7a: by phosphorylating light-activated cone opsin it starts opsin shut-off and so recovery of the cone photoresponse. The knockout effect is modest because cone Grk1b also contributes; that is partial redundancy with the rod/cone paralog Grk1b, not with grk7b (larval Grk7 protein is undetectable in the grk7a knockout, so grk7b does not substitute here). Supporting Evidence: PMID:30372740 A logarithmic linear regression of the non-saturating portion of the response shows a recovery half-life of 1.7 seconds for wildtype larvae, versus 2.3 and 3.0 seconds for grk1b−/− and grk7a−/− larvae, respectively (Fig. 4B, inset). PMID:30372740 Analysis of grk7a−/− larvae at 5 dpf show undetectable levels of Grk7 compared to wildtype (Fig. 2A, 2B). |
| GO:0036368 photoresponse recovery | IMP PMID:36273582 Grk7 but not Grk1 undergoes cAMP-dependent phosphorylation i... | ACCEPT | Summary: In the grk7a knockout, forskolin (raised cAMP) no longer delays cone photoresponse recovery, whereas it still does in grk1b knockouts; Grk7a is phosphorylated in a cAMP/PKA-dependent way in the dark. Reason: Confirms that Grk7a acts in cone photoresponse recovery and shows that it is the cAMP-regulated component of that step. Supports the core process. Supporting Evidence: PMID:36273582 These data suggest that the delay in cone recovery brought about by increased levels of cAMP is mediated by Grk7a rather than Grk1b in zebrafish larvae. |
| GO:0050254 rhodopsin kinase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from the GRK7 node (PTN002806910), seeded by human GRK7 and the two zebrafish paralogs (grk7a itself, ZDB-GENE-050824-1, and grk7b). Reason: grk7a appears in the WITH/FROM because it has its own IDA for this activity; that is expected and shows the target is experimentally grounded. The activity is ancestral to all GRK7s and retained by grk7a with very high Vmax (PMID:16787417). Supporting Evidence: PMID:16787417 The recombinant GRKs phosphorylated light-activated rhodopsin, and the Vmax value of the major cone subtype, GRK7-1, was 32-fold higher than that of the rod kinase, GRK1A. |
| GO:0050254 rhodopsin kinase activity | IDA PMID:16787417 GRK1 and GRK7: unique cellular distribution and widely diffe... | ACCEPT | Summary: Wada et al. 2006 expressed recombinant zebrafish GRKs and measured phosphorylation of light-activated rhodopsin. GRK7-1 (grk7a; UniProt synonym grk7-1, EMBL AB212995) had a Vmax 32-fold higher than rod GRK1A (UniProt: KM 4.4 uM, Vmax 773 nmol/min/mg). Reason: Direct biochemical evidence for the core molecular function. Abstract-only in cache; the gene mapping (GRK7-1 = grk7a) is from the UniProt synonym and the EMBL accession. A later paper (Shen et al. 2021), citing this study, states that GRK7-2 (grk7b) is more than 30-fold slower (PMID:33579376), so the high activity is a property of grk7a specifically. Supporting Evidence: PMID:16787417 The recombinant GRKs phosphorylated light-activated rhodopsin, and the Vmax value of the major cone subtype, GRK7-1, was 32-fold higher than that of the rod kinase, GRK1A. PMID:33579376 A previous comparison of the initial rates of rhodopsin phosphorylation by recombinant GRKs in zebrafish revealed that rhodopsin phosphorylation activity of GRK7a (GRK7–1) is more than 30-fold faster than those of GRK1a, GRK1b, and GRK7b (GRK7–2 [29]). |
| GO:0050254 rhodopsin kinase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Rhea/EC-based rhodopsin kinase activity (EC 2.7.11.14), agreeing with the experimental annotation. Reason: Consistent with the IDA (PMID:16787417) and the IBA. Supporting Evidence: PMID:16787417 The recombinant GRKs phosphorylated light-activated rhodopsin |
| GO:0120199 cone photoreceptor outer segment | IDA PMID:16787417 GRK1 and GRK7: unique cellular distribution and widely diffe... | NEW | Summary: Proposed location. In situ hybridization and immunohistochemistry place GRK7-1 (grk7a) in cone outer segments, where the cone opsin substrate resides. Reason: The only location in GOA is the generic membrane. The cone outer segment is the compartment in which grk7a acts on light-activated opsin, and it is shown directly for GRK7-1 (PMID:16787417). Knockout immunostaining confirms that the larval retinal Grk7 signal is Grk7a (PMID:30372740). Supporting Evidence: PMID:16787417 In situ hybridization and immunohistochemical studies localized both GRK1B and GRK7-1 in the cone outer segments PMID:30372740 Immunocytochemical analysis also confirms knockout of Grk7 expression in grk7a−/− larvae, with no anti-Grk7 immunoreactivity compared to wildtype (Fig. 2C, middle column). |
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Download this section (compressed HTML)Q: grk7a and grk7b differ in rhodopsin-phosphorylation rate by more than 30-fold (PMID:33579376 citing PMID:16787417). Is this a real difference in catalytic efficiency on the native cone opsins (for grk7b, UV opsin opn1sw1), or an artefact of using bovine rhodopsin as substrate?
Suggested experts: photoreceptor biochemists
Q: Should ZFIN's IMP "photoreceptor activity" annotation from Rinner et al. 2005 be retired? The phenotype is already captured by phototransduction and photoresponse recovery, and grk7a's MF is rhodopsin kinase activity.
Suggested experts: ZFIN curators
Q: Is grk7b's UV-cone-restricted expression derived (lost from other cone types after the TGD) or ancestral? What is the cone-subtype expression of the single GRK7 of spotted gar?
Experiment: Express grk7b coding sequence under a pan-cone promoter (e.g. gnat2) in grk7a knockout larvae and measure paired-flash cone ERG recovery and forskolin sensitivity; compare with a grk7a transgene.
Hypothesis: grk7b cannot replace grk7a in the larval cones because it is not expressed there, not because its protein is inadequate.
Type: cross-rescue
Experiment: Measure grk7b and grk1b mRNA and protein (paralog-specific antibodies) in grk7a PTC mutants and in a promoter-deletion (RNA-less) grk7a allele, and test grk7a;grk1b and grk7a;grk7b double mutants for UV-cone versus other-cone recovery.
Hypothesis: The mild grk7a knockout phenotype reflects redundancy with grk1b rather than compensation by grk7b.
Type: double mutant and transcriptional adaptation test
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