The Journal of Cell Biology, Vol 80, 642-650, Copyright © 1979 by The Rockefeller University Press
Ligatin from embryonic chick neural retina
ER Jakoi and RB Marchase
Ligatin, a filamentous protein previously found in suckling rat ileum, has
been purified from plasma membranes of embryonic chick neural retina. The
isolated plasma membranes are covered in part by 4.5-nm filaments that can
be released from the membranes by treatment with Ca++. Subsequent dialysis
against EGTA followed by sieve chromatography results in purification of
the 10,000-dalton ligatin monomer. When labeled either with radioisotopes
or with fluorescamine, the monomer is shown to electrophorese as a single
discrete band in polyacrylamide gels. However, during standard fixing and
staining procedures it diffuses from the gels and thus is not visualized.
Ligatin's amino acid composition is distinguished by its high content of
polar residues, especially Glx and Asx, and by the presence of
phosphorylated serine. Upon re-addition of Ca++, purified ligatin monomers
polymerize to form filaments 3 nm in Diam, identical to those formed by
purified ileal ligatin. However, in both retina and ileum, the filaments
observed on plasma membranes are greater than 3 nm in Diam. In ileum, this
enlargement results from ligatin's function as a baseplate for the
attachment of another protein, a beta-N-acetylhexosaminidase, to the cell
surface. In retina, a corresponding difference in diameter between
filaments seen in vivo and those formed from repolymerized ligatin alone
and the co-solubilization of other proteins with ligatin suggest that
ligatin may also function there as a baseplate for other cell surface
proteins. The proteins associated with ligatin in retina differ
morphologically from beta-N-acetylhexosaminidase and do not possess this
enzymatic activity.
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