At the C-terminus of all known 4.1 proteins is a sequence domain unique to
these proteins, known as the C-terminal domain (CTD). Mammalian CTDs are as
sociated with a growing number of protein-protein interactions, although su
ch activities have yet to be associated with invertebrate CTDs. Mammalian C
TDs are generally defined by sequence alignment as encoded by exons 18-21.
Comparison of known vertebrate 4.1 proteins with invertebrate (Caenorhabdit
is elegans and Drosophila melanogaster) 4.1 proteins indicates that mammali
an 4.1 exon 19 represents a vertebrate adaptation that extends the sequence
of the CTD with a Ser/Thr-rich sequence. The CTD was first described as a
22/24-kDa domain by chymotryptic digestion of erythrocyte 4.1 (4.1R) [Leto,
T.L. & Marchesi, V.T. (1984) J. Biol. Chem. 259, 4603-4608]. Here we show
that in 4.1R the 22/24-kDa fragment is not stable but rapidly processed to
a 15-kDa fragment by chymotrypsin. The 15-kDa fragment is extremely stable,
being resistant to overnight digestion in chymotrypsin on ice. Analysis of
this fragment indicates that it is derived from residues 709-858 (SwissPro
t accession no. P48193), and represents the CTD of 4.1R. The fragment behav
es as a globular monomer in solution. Secondary-structure predictions indic
ate that this domain is composed of five or six beta strands with an alpha
helix before the most C-terminal of these. Together these data indicate tha
t the CTD probably represents an independent folding structure which has ga
ined function since the divergence of vertebrates from invertebrates.