Spcs2

COMPLEX ACTIVITY TRANSFER

Review rationale: SPCS2 is a noncatalytic accessory subunit of the signal peptidase complex; SEC11A or SEC11C carries the catalytic Ser-His-Asp triad. Neither canonical mouse Spcs2 nor human SPCS2 carries a peptidase activity annotation, and the PANTHER family review limits the family role to complex participation. The selected product additionally lacks the transmembrane architecture of the subunit. Catalysis by the complex does not establish peptidase activity of this protein.

Review rationale: GO:0006465 is obsolete in current GO (replaced by GO:0051604 protein maturation); the process is carried at the gene level by canonical Spcs2 (Q9CYN2) as GO:0016485 protein processing, so the prediction is not novel for the gene. SPCS2 is a noncatalytic subunit; catalysis is performed by SEC11A or SEC11C, so no catalytic step is lost. Its structurally resolved contribution is through its transmembrane domains, which with those of the other subunits form the bilayer window. A0A140LHW5 shares only canonical residues 1-66, within the N-terminal cytoplasmic domain (canonical 2-86), and lacks both transmembrane helices (87-107 and 112-132) and the C-terminal cytoplasmic domain, so it cannot contribute that membrane structure. The known mechanism does not, however, account for the full SPCS2 contribution to the process, and a binding or scaffolding role of the retained cytosolic segment toward complex or translocon components is neither shown nor excluded. There is no positive refutation, and whether the 74-residue product exists as a stable protein is also unestablished, so the prediction cannot be validated or refuted for this product.

DOMAIN ARCHITECTURE MISMATCH

Review rationale: Canonical Spcs2 (Q9CYN2) carries GO:0005787 (IBA, IEA, ISO), accepted in its review on human structural and yeast genetic evidence, so the term is not novel for the gene. SPCS2 is an integral two-pass ER membrane subunit of the complex, and its structurally resolved role in the complex is through its transmembrane region, which with the other subunits forms the membrane window. A0A140LHW5 retains only the N-terminal cytosolic residues 1-66 of the 226-residue subunit, followed by an unrelated eight-residue tail, and lacks both transmembrane helices (canonical 87-107 and 112-132), so it lacks the membrane region through which SPCS2 is a component of the signal peptidase complex.