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90. McLean, R. J., Katz, K. R., Sedita,
N. J., Menoff, A. L., Laurendi, C. J., and R. M. Brown, Jr. 1981.
Dynamics of concanavalin A binding sites in Chlamydomonas moewusii
flagellar membranes. Ber. Deutsch. Bot. Ges. Bd. 94:387-400.
90. Introduction
Chlamydomonas
is a single-celled, biflagellated, photosynthetic alga that can
be grown axenically in a simple salts solution in the light.
Many species that exist in culture have two mating types, usually
designated (+) and (-). Both mating types of Chlamydomonas
can be maintained separately in an actively growing, motile state
(vegetative cells) in liquid media. The mating types have no
affinity for each other at this stage. Upon transfer to a medium
deficient in nitrogen and low in other salts (induction medium)
the vegetative cells differentiate into gametes without morphological
change. Flagellar membrane differentiation is the main detectable
difference between vegetative cells and gametes. Differentiation
can be tested by mixing cells with previously differentiated gametes
of the opposite type to observe clumping or agglutination of cells
which is caused by adhesion of flagellar membranes. In the case
of C. moewusii, a fusing pair of opposite gametes
is released eventually from the clump with their flagellar membranes
unattached to each other and lacking an affinity for other free-swimming
gametes (LEWIN 1956, TRAINOR 1959, WIESE 1969). A summary diagram
of the Chlamydomonas moewusii sexual cycle is presented
in Figure 1. The isoagglutination of Chlamydomonas gametes
by concanavalin A (Con A) was first demonstrated by WIESE and
SHOEMAKER (1970). This tetravalent lectin attached to the flagellar
membranes to cause a common bond between cells. We later reported
(MCLEAN and BROWN 1974) that although tetravalent Con A could
isoagglutinate gametes and prevent their interaction with opposite
gametes, the monovalent form could do neither. Treatment of gametes
with trypsin would eliminate their mating adhesiveness but not
their ability to agglutinate in the presence of Con A. This suggested
that the adhesive mating sites and the Con A binding sites were
not the same. Subsequent reports have demonstrated the presence
of multiple sites associated with the flagellar membrane of Chlamydomonas
(CLAES 1975, SOLTER et al. 1977, FOREST and GOODENOUGH 1977, ADAIR
and GOODENOUGH 1978, ADAIR et al. 1979, MONK et al. 1979 BLOODGOOD
1979). The work of BLOODGOOD (1977, 1979) and co-workers (1979)
is one of the more interesting breakthroughs in the area of flagellar
membrane research and its effect on our understanding of the Chlamydomonas
mating reaction. They reported on the movement of membrane binding
sites observed by the attachment of polystyrene microspheres,
in a bidirectional fashion on the flagella of Chlamydomonas.
This movement, which was parallel to the long axis, was observed
on both gametes and vegetative cells. It is rapid and observable
without time lapse photography. Two different binding sites may
pass one another while moving in opposite directions (BLOODGOOD
1977). The flagellar surface motility can be reversibly inhibited
by high sodium or potassium, lowered temperature, or lowered calcium
concentration. Inhibition of protein synthesis results in loss
of microsphere binding and surface motility (BLOODGOOD 1979).
Binding sites can be removed from flagella by pronase treatment,
but are replaced in the presence of new protein synthesis. BLOODGOOD
(1979) Further reported that there is a normal turnover of membrane
proteins including those involved in marker adhesion and motility.
The turnover is thought to occur by the release of membrane vesicles
into the medium (MCLEAN et al. 1974, BERGMAN et al. 1975, SNEI.L
1976). Recently, SNELL and MOORE (1980) reported that rapid turnover
of adhesive sites occurred in response to cellular adhesion while
turnover was slower in the absence of it. Other evidence of binding
site movement has been reported in Chlamydomonas by GOODENOUGH
and JURIVICH (1978). This movement seems to be a result of gametic
differentiation since only gametes were observed to move antibody
toward the distal end or tip of the flagella when presented with
antisera against gametic or vegetative flagella of Chlamydomonas
reinhardtii. The resulting agglutination of gametes occurs
at the flagellar tips whereas adhesion of vegetative cells to
one another by antisera can be seen along the entire length of
the flagella. Although Con A has continued to be used as a tool
in the study of other cell systems, its use has been more limited
in this cell system. We present here a further study of Con A
isoagglutination, distribution and movements of binding sites
on the flagellar membrane, and a discussion of what this means
to the Chlamydomonas mating reaction.
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