Silaffin-1 precursor, the founding member of the silaffin family of diatom biosilica-forming polypeptides. The 265-residue precursor carries a cleaved signal peptide, an acidic N-terminal propeptide of uncertain function, and a C-terminal array of seven lysine/serine-rich repeats that is proteolytically processed at pairs of basic residues into the mature mineralization peptides silaffin-1A1 (five copies), silaffin-1A2 and silaffin-1B. The mature peptides are among the most heavily post-translationally modified polypeptides known: specific lysines carry long poly(N-methylpropylamine) chains, others are dimethylated or converted to trimethyl-delta-hydroxylysine, and in the native state every serine of silaffin-1A1 is phosphorylated, whereas silaffin-1A2 is only minimally phosphorylated. The resulting zwitterionic peptides self-assemble and precipitate networks of silica nanospheres from monosilicic acid within seconds in vitro; the lysine modifications strongly influence that activity, and the phosphorylation is essential for it. In the cell the peptides are integral, HF-resistant components of the silica cell wall (frustule) and are thought to template and accelerate silica polycondensation during wall biogenesis, acting together with free long-chain polyamines to set silica sphere size and the species-specific wall pattern.
Curated functional classes representing distinct biological activities. These may be splice variants, cleavage products, or other forms with different functions.
SIL1_1A1PRO_0000032602 (residues 163-177) PRO_0000032604 (residues 182-196) PRO_0000032606 (residues 201-215) PRO_0000032608 (residues 220-234) PRO_0000032610 (residues 239-253) SIL1_1A2PRO_0000032600 (residues 141-158) SIL1_1BPRO_0000032598 (residues 108-136) Definition: Binding silicic acid and promoting its condensation into solid silica (polymerized silicon dioxide), thereby accelerating silica formation and determining the size and morphology of the silica particles formed. Carriers of this function are typically not classical enzymes but polycationic, post-translationally modified polypeptides or peptide assemblies that provide a charged template surface for nucleation and growth.
Justification: SIL1 (silaffin-1) has no GO molecular function term that fits. Searching GO for "silica" returns only GO:0034021 response to silicon dioxide and GO:0071251 cellular response to silicon dioxide, both response terms; the only other silica-related function is GO:0015115 silicate transmembrane transporter activity, which is a transporter and plainly wrong for a cell-wall structural peptide. The demonstrated activity of the sil1 gene products is the rapid conversion of monosilicic acid into networks of silica nanospheres, which is a molecular function in its own right and is the single most-studied property of this protein family. Per the CLAUDE.md rule on activities with no GO term, the core function above leaves molecular_function unset and points at this proposed name rather than forcing an ill-fitting id. Note that the term is deliberately not called a catalytic or lyase-type activity: although the UniProt FUNCTION line says "Catalyzes the polymerization of silica spheres", no turnover number, active site or catalytic mechanism has been demonstrated, and the primary literature describes the effect as acceleration and templating by a charged surface.
Parent term: molecular_function
Supporting Evidence:
Definition: A membrane-bounded, acidified intracellular vesicle of silicifying organisms, notably diatoms, within which silicic acid is concentrated and condensed into solid biosilica, and whose expansion and shaping determine the morphology of the resulting silicified cell wall element.
Justification: The silica deposition vesicle (SDV) is the defining organelle of diatom biosilicification and the compartment in which silaffins are proposed to act, but GO has no term for it: an OLS search of the go ontology for "silica deposition vesicle" returns zero classes. Without this term, the intracellular site of silica formation cannot be annotated for any diatom protein, and curators are forced either to omit the location or to misuse a generic secretory vesicle term. This review therefore annotates only the mature biosilica location (GO:0030312) and records the SDV as an ontology gap.
Parent term: cytoplasmic vesicle
Supporting Evidence:
Definition: The silicified external encapsulating structure of a diatom, composed chiefly of amorphous hydrated silica together with an occluded organic matrix of silaffins, long-chain polyamines and associated proteins, and comprising two overlapping valves joined by girdle bands.
Justification: GO:0005618 cell wall carries an explicit "Not to be used for manual annotation" instruction directing curators to a lineage-specific child, but the children it offers cover only bacteria (GO:0009274), plants (GO:0009505), fungi (GO:0009277) and archaea (GO:0030115 S-layer). There is no child for the diatom frustule, which is chemically and developmentally unlike all four. As a result this review annotates the location at GO:0030312 external encapsulating structure, which is one level too general for a structure as well characterized as the diatom frustule. A diatom-specific child of GO:0005618 would let silaffins, frustulins, pleuralins and silicic acid transporters all be annotated at the right granularity.
Parent term: cell wall
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0030312 external encapsulating structure | IDA PMID:11106386 Species-specific polyamines from diatoms control silica morp... | NEW | Summary: Silaffin-1 peptides are integral components of the diatom silica cell wall, so tightly incorporated into the biosilica that releasing them requires dissolving the wall in hydrogen fluoride. Reason: This is a direct, physical demonstration of location: the peptides are recovered from purified cell walls and cannot be extracted without destroying the mineral phase, which is stronger evidence of wall residence than a typical co-localization assay. GO:0030312 rather than its child GO:0005618 cell wall is used deliberately: GO:0005618 carries a "Not to be used for manual annotation" instruction pointing curators to a lineage-specific child, and the children GO offers (bacterial, plant-type, fungal-type, archaeal S-layer) contain no option for a diatom frustule. The missing diatom child is filed under proposed_new_terms. No intracellular location is proposed: the silica deposition vesicle is where silica formation is believed to occur and UniProt's PTM note invokes it, but the evidence available for this protein demonstrates only the mature biosilica location, and GO has no SDV term in any case. Supporting Evidence: PMID:11106386 Diatom cell walls were isolated from six different species, C. fusiformis , N. angularis, C. didymum , C. debilis , E. zodiacus , and S. turris (see Materials and Methods ), and were dissolved in anhydrous HF. PMID:11106386 Compared with the silaffin pattern obtained from C. fusiformis (Fig. 1 , lane 1), it is quite evident that each diatom species contains a species-specific set of polypeptides. PMID:11349130 Two silica-precipitating peptides, silaffin-1A(1) and-1A(2), both encoded by the sil1 gene from the diatom Cylindrotheca fusiformis, were extracted from cell walls and purified to homogeneity. |
| GO:0005199 structural constituent of cell wall | IC PMID:12386330 Self-assembly of highly phosphorylated silaffins and their f... | NEW | Summary: Silaffin-1 peptides are proposed to be a structural part of the diatom wall, not only located in it: native silaffins form a plastic silaffin-silica phase in vitro that the authors suggest may be a building material of the biosilica, and the peptides are retained inside the mineral of the mature wall. Reason: This is a curator inference (IC), not a direct demonstration, and is stated as such. No experiment has measured a contribution of SIL1 to the mechanical or structural integrity of the wall; there is no loss-of-function evidence in the living diatom (see suggested_experiments). The inference combines two lines. (a) PMID:12386330 reports that native, phosphorylated silaffins form a plastic silaffin-silica phase during silica morphogenesis in vitro, which the authors say "may represent" a building material for diatom biosilica; the hedge is the authors' and is kept here. (b) The peptides are retained within the mineral of the mature wall and can be released only by dissolving it in HF, consistent with their being part of the wall's organic matrix rather than a surface-adsorbed component. Line (b) on its own is a location fact, and it is the evidence for the GO:0030312 row above; it is used here only as corroboration of (a). This row captures the proposed structural half of silaffin-1's molecular role. The other half, the silica-precipitating activity, has no GO term and is proposed rather than annotated (see proposed_new_terms "silicic acid polycondensation activity"). Because the structural claim rests on a hedged in vitro observation, this row should be revisited if in vivo data on wall integrity become available. Supporting Evidence: PMID:12386330 Time-resolved analysis of silica morphogenesis in vitro detected a plastic silaffin-silica phase, which may represent a building material for diatom biosilica. PMID:11106386 Diatom cell walls were isolated from six different species, C. fusiformis , N. angularis, C. didymum , C. debilis , E. zodiacus , and S. turris (see Materials and Methods ), and were dissolved in anhydrous HF. |
| GO:0042546 cell wall biogenesis | IDA PMID:10550045 Polycationic peptides from diatom biosilica that direct sili... | NEW | Summary: The mature silaffin-1 peptides themselves carry out the silica precipitation step that builds the diatom cell wall, converting monosilicic acid into networks of silica nanospheres within seconds. Reason: The participation test in CLAUDE.md is satisfied directly rather than by necessity evidence: purified silaffin-1 peptides, acting alone on a monosilicic acid solution, perform the condensation that produces the wall's mineral phase. SIL1 is the entity doing the work, not something the process merely requires or consumes. Note what is deliberately NOT claimed. GO:0031214 biomineral tissue development is the obvious "biomineralization" term but is is_a GO:0009888 tissue development with ancestry through multicellular organism development; a unicellular diatom has no tissue, so that term is wrong here despite the apparent subject-matter match. Equally, no polyamine- or methylation-related process term is proposed: those modifications are installed ON SIL1 by other, still unidentified enzymes, so SIL1 is the substrate of that chemistry, which is precisely the non-participation case. The remaining caveat, recorded as a suggested experiment, is that all the functional evidence is in vitro precipitation by purified peptides; there is no loss-of-function evidence in the living diatom. Supporting Evidence: PMID:10550045 A set of polycationic peptides (called silaffins) isolated from diatom cell walls were shown to generate networks of silica nanospheres within seconds when added to a solution of silicic acid. PMID:11349130 In vitro, both peptides precipitate silica nanospheres within seconds when added to a monosilicic acid solution. file:CYLFU/SIL1/SIL1-uniprot.txt formation of silica cell wall of diatoms. |
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Download this section (compressed HTML)Q: Is the acidic N-terminal region of the SIL1 precursor (residues 20-107) a functional domain or only a propeptide? UniProt flags this explicitly as unknown. Given that native silaffin activity depends on a zwitterionic balance between polyanionic phosphoserine and polycationic polyamines, an acidic segment could plausibly act as an intramolecular chaperone or a self-assembly regulator that is removed on maturation, rather than being inert.
Suggested experts: Nils Kroeger, Manfred Sumper
Q: Which enzymes install the polyamine, methylation, hydroxylation and phosphorylation modifications on silaffins, and are they themselves silica deposition vesicle residents? These modifications, not the peptide backbone, carry the activity, yet none of the responsible enzymes is identified. They would be the real molecular-function targets for the polyamine-transfer and methylation chemistry; SIL1 is the substrate of those reactions, not their catalyst, so those activities must not be annotated to SIL1.
Suggested experts: Nils Kroeger, Rainer Deutzmann
Q: Should GO create a diatom-specific child of GO:0005618 cell wall, and a cellular component term for the silica deposition vesicle? Both gaps are recorded in proposed_new_terms, and both affect every silicifying organism in GO, not just this gene.
Experiment: Reconstitute silica precipitation with recombinant, chemically defined SIL1 peptides in which each modification class is installed or omitted combinatorially (polyamine only, phosphate only, both, neither), and measure precipitation rate and resulting sphere size distribution. The literature establishes that both modifications are required, but the two were demonstrated in separate studies on differently extracted material (HF-extracted versus native), so their individual and joint contributions to rate versus morphology have never been separated.
Hypothesis: The lysine-linked polyamines and the serine phosphorylation make separable contributions to silaffin activity, the former driving precipitation rate and the latter controlling self-assembly and resulting silica morphology.
Type: in vitro biochemical reconstitution
Experiment: Localize SIL1 peptides in vivo through the cell cycle by immuno-electron microscopy or a genetically encoded tag, specifically asking whether they are present in the silica deposition vesicle prior to mineral deposition. The SDV is where silica formation is believed to occur, and UniProt's PTM note invokes it, but the direct evidence available for this protein shows only the mature biosilica location, which is why no intracellular location is annotated in this review.
Hypothesis: SIL1 peptides enter the silica deposition vesicle before silica condensation begins, rather than associating with biosilica only after it has formed.
Type: subcellular localization / immuno-EM
Experiment: Knock down or knock out sil1 in Cylindrotheca fusiformis and characterize frustule morphology, silica content and nanosphere size by electron microscopy. All functional evidence for silaffin-1 is currently in vitro precipitation by purified peptides; there is no loss-of-function evidence that SIL1 is required for normal wall formation in the living diatom, which is the main gap between the accepted in vitro activity and the asserted in vivo role in cell wall biogenesis.
Hypothesis: SIL1 is required in vivo for normal frustule silicification, and its loss alters silica nanosphere size or wall patterning.
Type: reverse genetics / phenotypic analysis
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