Maspin (SERPINB5) Is an Obligate Intracellular Serpin

Abstract

Maspin (SERPINB5) is a tumor suppressor lost in breast and prostate cancer whose molecular function is unknown. It is a non-inhibitory member of the clade B serpins suggested to play a role in a plethora of intracellular and extracellular settings, yet its normal cellular distribution has never been clarified. Here we investigate the distribution of maspin in non-transformed human epithelial cells. By indirect immunofluorescence, maspin has a nucleocytoplasmic distribution in breast (MCF10A) and prostate (RWPE-1) cells and, by immunoblotting and pulse-chase analyses, is neither glycosylated nor secreted. Cell surface biotinylation studies also show that maspin is not present at the cell surface. Differentiation of MCF10A cells into three-dimensional acini results in the redistribution of maspin from the nucleus to the cytoplasm but does not result in secretion. Addition of an efficient conventional signal peptide to maspin directs it into the secretory pathway and results in glycosylation but not secretion. We further show that maspin in the cytoplasm of MCF10A cells is a soluble monomeric protein that is not detectably associated with the cytoskeleton or other extractable components. Taken together, these results suggest that maspin is restricted to an intracellular, possibly nuclear, role in which it influences cell-matrix interactions indirectly. It is probably released only as a consequence of cell damage or necrosis.

Proteolytic Histone Modification by Mast Cell Tryptase, a Serglycin Proteoglycan-dependent Secretory Granule Protease

“…Due to this organization, tryptase is resistant to all known macromolecular protease inhibitors found in mammals (23). The enzyme is thereby well suited for carrying out proteolytic processes in the cellular environment, unhindered by the multitude of protease inhibitors of serpin type that are abundant within the cytosol and also in the nucleus (24,25). The ability of tryptase to degrade histones intracellularly is thus compatible with its biochemical organization.…”

Cervicovaginal microbiome dysbiosis is associated with proteome changes related to alterations of the cervicovaginal mucosal barrier

“…Minor modifications were made to GO-term GO:0030414 (excluding complement factor 3 and serpin B5), and GO:0005856 (excluding proteasome subunit beta type-3), because the functions of the excluded proteins are better described by other terms according to the published literature. [53][54][55] An annotated heatmap was drawn using the ''Heatplus'' package in R and hierarchical clustering with Euclidean distance and complete linkage was performed on the range-scaled ion intensities of the differentially abundant proteins.…”

The Aggregation-Prone Intracellular Serpin SRP-2 Fails to Transit the ER inCaenorhabditis elegans

“…The signal peptide directed sSRP-2::GFP to the ER where it appeared to form large inclusions, much like sGFP::ATZ, that also colocalized with the sDsRed::KDEL marker (Figure 2, P-T). This result was not surprising, as at least two members of the clade B serpin family, MASPIN/SERPINB5 and SERPINB6, appear to be obligate intracellular proteins and accumulate in the ER as endo-H-sensitive aggregates when they are forced into the secretory pathway by a synthetic signal peptide (Scott et al 1996;Teoh et al 2010). Moreover, SER-PINB6 no longer maintains its inhibitory activity when extracted from the ER.…”

“…Although some of these secreted intracellular serpins possess complex N-linked carbohydrates suggestive of Golgi processing, secretion still occurs in the presence of tunicamycin (Keller and Swank 1978) and may involve unconventional signal pathway(s) similar to those used by FGF-2 and IL-1b (Nickel and Rabouille 2009;Giuliani et al 2011;Malhotra 2013). Indeed, forced expression of wild-type protease inhibitor 6 (PI6)/SERPINB6 and MASPIN/SERPINB5 into the ERGolgi pathway by fusing an N-terminal signal peptide leads to overt polymerization and ER retention (Scott et al 1996;Teoh et al 2010). None of the 12 other human clade B serpins appear to be secreted.…”