Evolution of Insulin and Insulin 
Receptor Molecules 
The coevolution of insulin, IGFs, and their recep- 
tor molecules is currently being explored in both 
vertebrate and nonvertebrate species. Complemen- 
tary DNAs encoding prepro-IGF from the hagfish and 
lamprey (jawless vertebrates) have been identified 
and sequenced. The IGF molecules from these prim- 
itive vertebrates are intermediate in structure be- 
tween those of human IGF-1 and -II, suggesting that 
they predate the duplication and divergence of the 
IGF gene. However, a number of features have been 
conserved, including the presence in lamprey 
serum of IGF-binding proteins. In amphioxus, a sim- 
ple ancestral chordate, a preproinsulin-like mole- 
cule having features of both vertebrate insulins and 
IGFs has been identified. This insulin/IGF hybrid 
molecule may represent an intermediate stage in the 
divergence of the IGFs from an ancestral "insulin" 
gene encoding a more typical preproinsulin-like 
protein. Dr. Steiner and his colleagues are also char- 
acterizing the cognate receptors for the above- 
mentioned hormones by cDNA cloning and in vitro 
expression. Comparison of insulin and IGF recep- 
tors in primitive vertebrates should help to identify 
important structural features that are conserved 
for specific ligand binding and transmembrane sig- 
naling. 
Evolution of prohormone convertases. The re- 
cent identification of several mammalian propro- 
tein convertases has led to interest in their possible 
origin and diversification from an ancestral subtili- 
sin-related protease similar to the yeast enzyme 
Kex2. These enzymes include furin, a widely ex- 
pressed Golgi-associated protease that appears to 
process a variety of growth and blood-clotting fac- 
tor precursors, PC2 and PC3/PC1, prohormone- 
processing enzymes expressed only in neural and 
endocrine tissues, as well as several other new 
members of this subtilisin-related superfamily. 
Kex2 and furin are both Golgi associated via trans- 
membrane domains and thus seem likely to fulfill a 
variety of secretory pathway processing functions. 
The more-specialized PC2 and PC3 enzymes that 
lack membrane anchors and function only in special- 
ized secretory granules in neuroendocrine tissues 
appear likely to be later innovations. Dr. Steiner and 
his colleagues have identified several PC-like en- 
zymes in amphioxus; the initial findings indicate 
these enzymes have been highly conserved in verte- 
brate evolution. They have also identified a PC3-like 
enzyme in the simple metazoan Hydra vulgaris, a 
coelenterate. This finding is of special interest inas- 
much as hydra is the simplest organism known to 
have specialized neural cells secreting neuropep- 
tides related to some of those in more complex 
eukaryotes. 
Proteolytic Maturation of the Insulin 
Receptor Precursor 
Recent studies from Dr. Steiner's laboratory have 
focused on the proteolytic cleavage of the a//?- 
proreceptor molecule. A mutagenesis study indi- 
cated that only the arginine residues 1 and 4 on ei- 
ther side of the tetrabasic cleavage site (R4K3R2R1I) 
are important for its recognition by the precursor- 
processing protease, while the internal residues 
(numbers 2 and 3) are less critical. These results are 
consistent with the possibility that the proreceptor 
is processed by a Golgi-associated protease such as 
furin. The presence of exon 1 1 allows the expressed 
proreceptor (mutated at the cleavage site) to be al- 
most fully functional in its binding and tyrosine ki- 
nase activity, while proreceptor molecules lacking 
the 12-amino acid sequence encoded by this small 
exon (expressed in most tissues) have greatly re- 
duced binding affinity and impaired autophosphor- 
ylation. (This research work has been supported by 
grants from the United States Public Health Service.) 
lAPP Expression in Normal Islets 
and Insulinomas 
Recent studies have shown that amyloid deposits 
in the islets of type II diabetics contain a neuropep- 
tide related to calcitonin gene-related peptide 
(CGRP). This 37-amino acid peptide (also known 
as amylin) is expressed in the ^ cells and is stored in 
the secretory granules with insulin. Structural varia- 
tions in the central region of LAPP appear to corre- 
late with species-dependent tendencies to form 
amyloid deposits in diabetic individuals. Studies on 
the biosynthesis and processing of prepro-lAPP and 
the expression of LAPP in islets and various islet cell 
tumor lines indicate that although insulin and lAPP 
are synthesized and released together from normal 
adult islets and i8TC3 tumor cells, the expression of 
LAPP in tumors appears to be up-regulated relative 
to insulin expression. Transgenic mice expressing 
human LAPP in the |8 cells are being studied as a 
possible model for the development of islet amyloid 
deposits in type II diabetics. (The work described 
above is supported by a grant from the LJnited States 
Public Health Service; the transgenic studies are a 
collaboration with Dr. Niles Fox at the Eli Lilly Re- 
search Laboratories.) 
Dr. Steiner is also the A.N. Pritzker Distin- 
guished Service Professor of Biochemistry and Mo- 
lecular Biology and Medicine at the University of 
Chicago. 
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