Ultrastructure and assembly of cellulose
terminal synthesizing complexes (terminal complexes, TCs) in the
alga Vaucheria hamata (Waltz) were investigated by high
resolution analytical techniques for freeze-fracture replication.
Vaucheria TCs consist of many diagonal rows of subunits
located on the inner leaflet of the plasma membrane. Each row
contains about 10-18 subunits. The subunits themselves are rectangular,
approx. 7 x 3.5 nm, and each has a single elliptical hole which
may be the site of a single glucan chain polymerization. The
subunits are connected with extremely small filaments (0.3-0.5
nm). Connections are more extensive in a direction parallel to
the subunit rows and less extensive perpendicular to them. Nascent
TC subunits are found to be packed within globules (15-20 nm in
diameter) which are larger than typical intramembranous particles
(IMPS are 10-llnm in diameter) distributed in the plasma membrane.
The subunits in the globule, which may be a zymogenic precursor
of the TC, are generally exhibited in the form of doublets. Approximately
6 doublets are connected to a center core with small filaments.
The globules are inserted into the plasma membrane together with
IMPS by the fusion of cytoplasmic (Golgi derived) vesicles. Two
or three globules attach to each other, unfold, and expand to
form the first subunit rows of the TC on the inner leaflet of
the plasma membrane. More globules attach to the structure and
unfold until the nascent TC consists of a few rows of subunits.
These rows are arranged almost parallel to each other. Two formation
centers of subunits appear at both ends of an elongating TC.
New subunits carried by the globules are added at each of these
centers to create new rows until the elongating TC structure is
completed. On the basis of this study, a model of TC assembly
and early initiation of microfibril formation in Vaucheria