Phospholipid Scramblase 1 Is Secreted by a Lipid Raft-dependent Pathway and Interacts with the Extracellular Matrix Protein 1 in the Dermal Epidermal Junction Zone of Human Skin

Abstract

We examined the interaction of ECM1 (extracellular matrix protein 1) using yeast two-hybrid screening and identified the type II transmembrane protein, PLSCR1 (phospholipid scramblase 1), as a binding partner. This interaction was then confirmed by in vitro and in vivo co-immunoprecipitation experiments, and additional pull-down experiments with GST-tagged ECM1a fragments localized this interaction to occur within the tandem repeat region of ECM1a. Furthermore, immunohistochemical staining revealed a partial overlap of ECM1 and PLSCR1 in human skin at the basal epidermal cell layer. Moreover, in human skin equivalents, both proteins are expressed at the basal membrane in a dermal fibroblast-dependent manner. Next, immunogold electron microscopy of ultrathin human skin sections showed that ECM1 and PLSCR1 co-localize in the extracellular matrix, and using antibodies against ECM1 or PLSCR1 cross-linked to magnetic immunobeads, we were able to demonstrate PLSCR1-ECM1 interaction in human skin extracts. Furthermore, whereas ECM1 is secreted by the endoplasmic/Golgi-dependent pathway, PLSCR1 release from HaCaT keratinocytes occurs via a lipid raft-dependent mechanism, and is deposited in the extracellular matrix. In summary, we here demonstrate that PLSCR1 interacts with the tandem repeat region of ECM1a in the dermal epidermal junction zone of human skin and provide for the first time experimental evidence that PLSCR1 is secreted by an unconventional secretion pathway. These data suggest that PLSCR1 is a multifunctional protein that can function both inside and outside of the cell and together with ECM1 may play a regulatory role in human skin.The human ECM1 gene (11 exons) is located on chromosome 1q21.2 (1, 2) and encodes four splice variants. ECM1a (without exon 5a) is expressed in basal keratinocytes, dermal blood vessels, and adnexal epithelia, including hair follicles and glands, whereas ECM1b, which lacks exon 7, is expressed in the spinous and granular layers of the epidermis (3-5). ECM1c was found in the basal layer of the epidermis (6), and a fourth splice variant results in a truncated protein of 57 amino acids (7). The ECM1 protein contains a 19-amino acid signal peptide followed by four domains: a cysteine-free N-terminal segment, two tandem repeats, and a C-terminal segment. The two tandem repeats and the C-terminal domain contain cysteines in a typical CC-(X 7-10 )C arrangement that is capable of forming protein double loops that could be involved in protein-protein interactions (1, 8). More recently, a rudimentary three-dimensional model divided the ECM1a protein into four distinct domains: an NH 2 -terminal domain forming ␣-helical structures, followed by three domains, whose amino acid sequences were highly comparable with the third domain of human serum albumin: SASDL2 (serum albumin subdomain-like 2), SASDL3, and SASDL4 (9).The function of ECM1 has not yet been elucidated in detail; however, it has been reported that ECM1 could act as a novel paracrine factor involved in the regulation of endocho...

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