Cleavage specificity studies show that SKI 1 generates a 28 kDa product from the 32 kDa brain derived neurotrophic factor precursor , cleaving at an RGLT downward arrowSL bond . ^^^ In vitro studies suggest that SKI 1 is a Ca2+ dependent serine proteinase exhibiting a wide pH optimum for cleavage of pro brain derived neurotrophic factor . . ^^^