Worm Breeder's Gazette 7(2): 21

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Ultrastructural Studies of Thick Filaments in unc-15 Mutants

A. Otsuka, D. Miller, I. Ortiz, H. Epstein

In order to examine the role of MHCA in Sup-3 suppression (see 
accompanying report), we hope to determine the composition of thick 
filaments in particular mutants by binding monoclonal antibodies to 
isolated filaments.  As a preliminary study, we have studied 
ultrastructure of thick filaments from several paramyosin mutants.
We have found that the 1.5 x 0.032 micron cigar-shaped filaments 
present in the paramyosin null mutant, unc-15(e1214)(ref.  1) are also 
found in unc-15(e73) strains.  In addition, the unc-15(e73) mutant 
contains 3.7 x 0.022  m filaments with tassle-like ends (wild type 
filaments are 10 to 14  m x 0.022  m).  The tassles probably represent 
protruding myosin heads and are similar to those found on the cigar-
shaped filaments.  Both cigar-shaped and longer tassled filaments 
contain myosin because both structures bind myosin heavy chain 
specific antibodies.  We have not been able to determine whether any 
of the structures observed correspond to the hollow filaments observed 
in unc-15(e73) animals.
The somewhat elongated and tassled filaments found in unc-15(e73) 
animals might account for their superior locomotion in comparison to 
unc-15(e1214) animals.  Perhaps the defective paramyosin is being 
incorporated into the tassled filaments of unc-15(e73) and, perhaps, 
these structures might be aberrant intermediates of thick filament 
assembly.  The possibility exists that these are abortive 
intermediates because animals containing the temperature-sensitive unc-
15(e1402) allele do not recover after late temperature shifts (2).  
However, the inability to recover from temperature shifting could be 
due to other requirements for muscle development.
The sup-3 mutation appears to allow defective paramyosin to assemble 
into thick filaments of normal appearance and length in sup-3(e1407) 
3) animals.  This result is consistent with the 
observation of thick filaments of normal appearance in cross sections 
of sup-3(e1407) 3) animals.  In addition to normal 
length filaments, the suppressed strain also contained the 3.7  m-long 
filaments as well as a few 1.5  m-long cigar-shaped filaments.