COMPARISON OF THE MOLECULAR-FORMS OF THE KEX2 SUBTILISIN-LIKE SERINE PROTEASES SPC2, SPC3, AND FURIN IN NEUROENDOCRINE SECRETORY VESICLES REVEALS DIFFERENCES IN CARBOXYL-TERMINUS TRUNCATION AND MEMBRANE ASSOCIATION/

The molecular forms and membrane association of SPC2, SPC3, and furin were investigated in neuroendocrine secretory vesicles from the anteri or, intermediate, and neural lobes of bovine pituitary and bovine adre nal medulla. The major immunoreactive form of SPC2 was the full-length enzyme with a molecular mass of 64 kDa. The major immunoreactive form of SPC3 was truncated at the carboxyl terminus and had a molecular ma ss of 64 kDa. Full-length 86-kDa SPC3 with an intact carboxyl terminus was found only in bovine chromaffin granules. Immunoreactive furin wa s also detected in secretory vesicles. The molecular masses of 80 and 76 kDa were consistent with carboxyl-terminal truncation of furin to r emove the transmembrane domain. All three enzymes were distributed bet ween the soluble and membrane fractions of secretory vesicles although the degree of membrane association was tissue specific and, in the ca se of SPC3, dependent on the molecular form of the enzyme. Significant amounts of membrane-associated and soluble forms of SPC2, SPC3, and f urin were found in pituitary secretory vesicles, whereas the majority of the immunoreactivity in chromaffin granules was membrane associated . More detailed analyses of chromaffin granule membranes revealed that 86-kDa SPC3 was more tightly associated with the membrane fraction th an the carboxyl terminus-truncated 64-kDa form.