J. Biol. Chem., Vol. 265, Issue 14, 8280-8284, May, 1990
Internal structure of ovomacroglobulin studied by electron microscopy
A Ikai, M Kikuchi and M Nishigai
Laboratory of Biodynamics, Tokyo Institute of Technology, Yokohama, Japan.
As a model for the molecular structure of proteins belonging to the alpha
2-macroglobulin family, ovomacroglobulin of reptilian origin was studied by
electron microscopy in the original tetrameric form as well as in the
dissociated forms into half- and quarter molecules. The following aspects
of the molecular internal structure which had previously not been known for
the homologous human alpha 2- macroglobulin or chicken ovomacroglobulin
were revealed. First, the negatively stained tetrameric native protein gave
an appearance of a collection of four semi-circular strings placed on the
four corners of a molecule. They were connected to each other in the center
of a molecule through a set of globular domains which formed a
cross-figured subunit contact region. Second, two kinds of active
half-molecules prepared either by the reduction of intersubunit disulfide
bonds or by the disruption of noncovalent subunit interface had similarly
elongated forms having semi-circular units on the two ends, indicating
quasi- equivalent subunit arrangement in the two kinds of half-molecules.
We thus concluded that the structure of native ovomacroglobulin can be
represented by four circular strings each equipped with an extra domain to
form the central intersubunit contact region. The results may also be
adapted to the internal structure of human alpha 2-macroglobulin because it
was sometimes possible to observe similar ring-like internal structure in
the human protein.
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