Fig. 4. CTSD stimulates transglutaminase 1 (TG1) activity in primary keratinocytes in vitro. Isolated membranes from HaCat-cells were left untreated (black squares), treated with pepstatin A (black circles), or with purified CTSD in the absence (black triangles) or presence of pepstatin A (white triangles) for 30 minutes and were subsequently measured for TG1 enzymatic activity by an in vitro enzyme assay using dimethylcasein and [1,4(n)-3H] putrescine as substrates. Results of three experiments performed in triplicate are shown (mean±s.e.m.).