A cytosolic chaperone complex controls folding and degradation of type III CD38

Cluster of differentiation 38 (CD38) is the best-studied enzyme catalyzing the synthesis of the Ca²⁺ messenger cyclic ADP-ribose. It is a single-pass transmembrane protein, but possesses dual orientations. We have documented the natural existence of type III CD38 in cells and shown that it is regulated by a cytosolic activator, calcium- and integrin-binding 1 (CIB1). However, how type III CD38 can be folded correctly in the reductive cytosol has not been addressed. Using the yeast two-hybrid technique with CD38's catalytic domain (sCD38) as bait, here we identified a chaperone, Hsp70-interacting protein (Hip), that specifically interacts with both the type III CD38 and sCD38. Immunoprecipitation coupled with MS identified a chaperone complex associated specifically with sCD38. Pharmacological and siRNA-mediated knockdown of Hsp90 chaperones decreased the expression levels of both sCD38 and type III CD38, suggesting that these chaperones facilitate their folding. Moreover, knockdown of Hsc70 or DNAJA2 increased the levels of both CD38 types, consistent with the roles of these proteins in mediating CD38 degradation. Notably, Hip knockdown decreased type III CD38 substantially, but only marginally affected sCD38, indicating that Hip was selective for the former. More remarkably, DNAJA1 knockdown decreased sCD38 but increased type III CD38 levels. Mechanistically, we show that Hsc70 mediates lysosomal degradation of type III CD38, requiring the lysosomal receptor Lamp2A and the C19-motif in the C terminus of CD38. Our results indicate that folding and degradation of type III CD38 is effectively controlled in cells, providing further strong support of its physiological relevance.