Cholesterol 3-sulfate interferes with cornified envelope assembly by diverting transglutaminase 1 activity from the formation of cross-links and esters to the hydrolysis of glutamine

The loss of transglutaminase 1 enzyme (TGase 1) activity causes lamellar ic hthyosis, Recessive X-linked ichthyosis (XI) results from accumulation of e xcess cholesterol 3-sulfate (CSO4) in the epidermis but the pathomechanism how elevated epidermal CSO4 causes ichthyosis is largely unknown. Here we p rovide evidence that III is also a consequence of TGase 1 dysfunction. TGas e 1 is a key component of barrier formation in keratinocytes: it participat es in the cross-linking of cell envelope (CE) structural proteins, and also forms the lipid bound envelope by esterification of long chain omega-hydro xyceramides onto CE proteins. Using involucrin and an epidermal omega-hydro xyceramide analog as substrates, kinetic analyses revealed that at membrane concentrations above 4 mol %, CSO4 caused a marked and dose-dependent inhi bitory effect on isopeptide and ester bond formation. Sequencing of tryptic peptides from TGase 1-reacted involucrin showed a large increase in deamid ation of substrate glutamines. me hypothesize that supraphysiological level s of CSO4 in keratinocyte membranes distort the structure of TGase 1 and fa oilitate the access of water into its active site causing hydrolysis of sub strate glutamine residues. Our findings provide further evidence for the pi votal role of the TGase 1 enzyme in CE formation.