Abstract
Insulin-like growth factor I (IGF-I) is a mitogenic peptide that is produced in most tissues and cell lines and plays an important role in embryonic development and postnatal growth. IGF-I is initially synthesized as a prohormone precursor that is converted to mature IGF-I by endoproteolytic removal of the carboxyl-terminal E-domain. Regulation of the conversion of proIGF-I to mature IGF-I is a potential mechanism by which the biological activity of this growth factor might be modulated. Endoproteolysis of the IGF-I prohormone occurs at the unique pentabasic motif Lys-X-X-Lys-X-X-Arg71-X-X-Arg-X-X-Arg. Recently, a family of enzymes which cleave prohormone precursors at sites containing multiple basic residues has been discovered. The goals of this study were 1) to determine which basic residues in the pentabasic proIGF-I processing site were necessary for proper cleavage and 2) to examine the role that subtilisin-related proprotein convertase 1 (SPC1/furin) might play in proIGF-I processing. We have shown that an expression vector coding for an epitope-tagged proIGF-I directs synthesis and secretion of mature IGF-I-(1-70), extended IGF-I-(1-76), proIGF-I, and N-glycosylated proIGF-I in human embryonic kidney 293 cells. Extended IGF-I-(1-76) is produced by cleavage at Arg77 and requires both Arg74 (P4) and Arg77 (P1). Cleavage at Arg77 does not occur in the SPC1-deficient cell lines RPE.40 and LoVo, suggesting that processing at this site is mediated by SPC1. Mature IGF-I-(1-70) is produced by cleavage at Arg71 and requires both Lys68 (P4) and Arg71 (P1). Lys65 in the P7 position is important for efficient cleavage. SPC1 is not required for processing at Arg71 since this cleavage occurs in RPE.40 and LoVo cells. These data suggest the existence of a processing enzyme which is specific for the Lys-X-X-Arg motif of proIGF-I.
Highlights
From the Wepartment of Biochemistry and Molecular Biology and the §Howard Hughes Medical Institute, University of Chicago, Chicago, Illinois 60637
IGF-I is initially synthesized as a prohormone precursor that is converted to mature IGF-I by endoproteolytic removal of the carboxyl-terminal Edomain
It is well established that most peptide hormones and growth factors are initially synthesized as biologically inactive precursors that are converted to active forms by endoproteolysis at specific sites
Summary
From the Wepartment of Biochemistry and Molecular Biology and the §Howard Hughes Medical Institute, University of Chicago, Chicago, Illinois 60637. Cleavage at Arg does not occur in the SPCI-deficient cell lines RPE. and LoVo, suggesting that processing at this site is mediated by SPCl. Mature IGF-I-(1-70) is produced by cleavage at Arg and requires both Ly s68 (P4) and Arg (PI). The sequences of human proIGF-IA and proIGF-IB are identical through the first 16 residues of the E-domain (4), including the unique pentabasic prohormone cleavage motif Lys-X-X-Lys-X-X-Arg X-X-Arg-X-X-Arg (Fig. 1). Cleavage is mediated by two subtilisin-related proprotein converting (SPC) enzymes, SPC2 (PC2) and SPC3 (PCl/3) (7, 8) These enzymes are serine proteases and have been shown to process several prohormones to mature hormones, including proglucagon (9) and the proopiomelanocortin precursor 0012). SPC4 has been reported to have a more strict requirement for a basic residue in the P2 position than does SPCl 06, 17)
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