PINB is a canonical long-loop PIN auxin exporter in the moss Physcomitrium patens (historically Physcomitrella patens). It acts partly redundantly with PINA in auxin distribution, gametophore development and shoot tropisms, and makes a prominent contribution to sporophyte development. PINB reaches the plasma membrane, but native-promoter PINB-GFP has a more even membrane distribution and stronger intracellular signal than PINA in protonemata; polarity depends on tissue and expression context. Native double-mutant export phenotypes and conserved PIN-family transport activity support its biochemical role. During early phyllid growth, PINA and PINB mainly regulate intracellular auxin levels rather than driving basipetal transport; basal polarity appears later in differentiated tip cells.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005783 endoplasmic reticulum | IBA GO_REF:0000033 | UNDECIDED | Summary: Native ER localization remains unresolved in its assay context. Reason: The ER IBD at PTN001589272 is a land-plant ancestral judgment seeded by Arabidopsis PIN5/6/8. Native moss PM localization does not exclude ER residence. Native-promoter PINB-GFP shows an intracellular pool (PMID:37528584), but the inspected experiment does not identify that pool as ER. The older PINA heterologous ER observation is not a PINB assay. No target-specific loss or erroneous ancestral placement is established. Keep the root-organelle claim unresolved, without replacement by plasma membrane. Propagation Review Root cause: UNRESOLVED Sources checked: PANTHER:PTN001589272 UNRESOLVED ER source node and Arabidopsis PIN5/6/8 descendants inspected; native target ER residence is not resolved by PM localization. Supporting Evidence: PMID:37528584 Notably, PpPINB-GFP did not show visible polarity at the tip of protonema but had a more even PM distribution with an increased intracellular signal (Supplemental Figure 5). PMID:25448003 Our results suggest that PINA and PINB act redundantly to remove auxin from the apex and initiating leaves, allowing normal development to proceed. |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Native moss PIN reaches the plasma membrane. Reason: PMID:37528584 examines the target GFP fusion under its native promoter in moss. PINB is more evenly distributed at the PM in protonemata, with intracellular signal; older constitutive-expression polarity must not be generalized to native expression. This independently supports the PAINT localization. The 2026 native phyllid time series (PMID:41984944) independently confirms membrane localization while restricting basal polarity to later differentiated tip cells, not early growing phyllids. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN001589272 SUPPORTS TRANSFER Native target GFP localization supports inheritance of PM localization. Supporting Evidence: PMID:37528584 Notably, PpPINB-GFP did not show visible polarity at the tip of protonema but had a more even PM distribution with an increased intracellular signal (Supplemental Figure 5). PMID:41984944 both PINA-GFP and PINB-GFP signals were primarily detected at membranes, without any clear basal polarization |
| GO:0016020 membrane | IEA GO_REF:0000120 | MODIFY | Summary: Membrane localization can be refined to plasma membrane. Reason: Native target localization supports the more precise GO:0005886, already represented in this review. This is refinement of generic membrane and does not remove a potentially separate ER pool. The 2026 native phyllid time series (PMID:41984944) independently confirms membrane localization while restricting basal polarity to later differentiated tip cells, not early growing phyllids. Proposed replacements: plasma membrane Supporting Evidence: PMID:37528584 Notably, PpPINB-GFP did not show visible polarity at the tip of protonema but had a more even PM distribution with an increased intracellular signal (Supplemental Figure 5). PMID:41984944 both PINA-GFP and PINB-GFP signals were primarily detected at membranes, without any clear basal polarization |
| GO:0022857 transmembrane transporter activity | IBA GO_REF:0000033 | MODIFY | Summary: The transporter activity is specifically auxin efflux. Reason: The general PAINT transporter function is correct and can be made substrate- and direction-specific. PMID:25448004 combines heterologous moss PIN-family transport assays with reduced export in native pinA pinB double mutants; a construct-specific PINB heterologous flux result has not been independently resolved from its supplement, so those biochemical assays are treated as family-level corroboration; PMID:25448003 supports the target-specific native redundancy. The 2024 H3-IAA assay tests PINA and MpPINZ, not PINB, and is not attributed to this protein. Refine to GO:0010329; native developmental phenotypes alone are not a direct turnover measurement. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN002065923 SUPPORTS TRANSFER The inspected PAINT node supports general transmembrane transport from Arabidopsis PIN descendant evidence; independent moss experiments justify substrate/direction refinement without rejecting ancestral transporter function. Proposed replacements: auxin efflux transmembrane transporter activity Supporting Evidence: url:https://folia.unifr.ch/documents/304040/files/gei_dat.pdf We assessed the auxin transport capabilities of these different PIN proteins by several means. url:https://folia.unifr.ch/documents/304040/files/gei_dat.pdf Also, a double mutant line in long moss PINs ( pinapinb) slightly reduced auxin export into the medium ( Figures 1 C, S1A–S1E, and S1H). PMID:25448003 Our results suggest that PINA and PINB act redundantly to remove auxin from the apex and initiating leaves, allowing normal development to proceed. |
| GO:0055085 transmembrane transport | IEA GO_REF:0000002 | MODIFY | Summary: Transmembrane transport is specifically auxin export. Reason: The PIN protein performs auxin export, so the generic BP can be refined to GO:0010315. Native pinA pinB export and complementary heterologous PIN-family transporter assays support this process (PMID:25448004); the latter are not claimed as a separately verified PINB flux measurement. Cell-to-environment export is not by itself proof of tissue-scale canalization, nor does it establish universal ABCB/AGC-kinase dependence. The 2026 phyllid study supports export-mediated control of intracellular auxin rather than early basipetal tissue transport; it does not negate exporter activity. Proposed replacements: auxin export across the plasma membrane Supporting Evidence: url:https://folia.unifr.ch/documents/304040/files/gei_dat.pdf Also, a double mutant line in long moss PINs ( pinapinb) slightly reduced auxin export into the medium ( Figures 1 C, S1A–S1E, and S1H). PMID:25448003 Our results suggest that PINA and PINB act redundantly to remove auxin from the apex and initiating leaves, allowing normal development to proceed. |
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Download this section (compressed HTML)Q: Does native PINB occupy a functional ER pool in particular tissues or physiological states, independently of transit to the plasma membrane?
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