Kalirin, a multifunctional PAM COOH-terminal domain interactor protein, affects cytoskeletal organization and ACTH secretion from AtT-20 cells

The production and regulated secretion of bioactive peptides require a seri es of lumenal enzymes to convert inactive precursors into bioactive peptide s plus several cytosolic proteins to govern granule formation, maturation, translocation, and exocytosis. Peptidylglycine alpha-amidating monooxygenas e (PAM), an enzyme essential for biosynthesis of many peptides, is an integ ral membrane protein with trafficking information in both its lumenal and c ytosolic domains. Kalirin, a PAM cytosolic domain interactor protein with s pectrin-like repeats and GDP/GTP exchange factor activity for Rad, is expre ssed with PAM in neurons but is not expressed in the anterior pituitary or AtT-20 corticotrope cells. Expression of Kalirin alters the cytoskeletal or ganization of Chinese hamster ovary and AtT-20 cells expressing membrane PA M. Expression of membrane PARI also alters cytoskeletal organization, demon strating the presence of endogenous proteins that can mediate this effect. Significant amounts of both PAM and Kalirin fractionate with cytoskeletal e lements. Since cytoskeletal organization is critical for exocytosis, consti tutive-like and regulated secretions were evaluated. Whereas the constituti ve-like secretion of adrenocorticotropic hormone (ACTH) is increased by exp ression of membrane PAM, regulated secretion is eliminated. Expression of K alirin in AtT-20 cells expressing membrane PAM restores stimulated secretio n of ACTH. Thus, Kalirin or its homologue may be essential for regulated se cretion, and the PAM-Kalirin interaction may coordinate intragranular with cytosolic events.