The role of two unconventional myosins, HUM-7 and HUM-6, in muscle structure and function Open Access
Wang, Yuchu (Spring 2026)
Abstract
In vertebrate striated muscle, the force of muscle contraction is transmitted to the outside of the cell via “costameres”, which are “integrin adhesion complexes” (IACs) that attach peripheral myofibrils to the cell membrane and extracellular matrix. In the striated muscle of C. elegans, IACs reside at sarcomeric M-lines and dense bodies, and at the muscle cell boundaries (MCBs). In screens for mutants with defective MCBs, the Benian lab identified the gene pix-1, which encodes a RacGEF (guanine nucleotide exchange factor) and the gene rrc-1, which encodes a RacGAP (GTPase activating protein). During the RacGAP screening, the Benian lab also found that hum-7 mutants have a defect at the MCB. HUM-7 is a class IX unconventional myosin. I have analyzed 9 hum-7 mutants. Two out of three missense mutations in the myosin motor domain, F319I and E583K display lack of accumulation of IAC proteins at the MCB, but two missense mutations in the RhoGAP domain, P1646L and R1564A display normal accumulation of IAC proteins. This suggests that the function of HUM-7 at the MCB requires its myosin motor but not its RhoGAP domain. A full-length deletion, syb10172, has reduced accumulation of IAC proteins at the MCB, and is slower moving. The two missense mutations in the myosin motor, F319I and E583K, also display reduced locomotion. CRISPR was used to insert an HA tag at the N-terminus of HUM-7. Using antibodies to HA, I can detect expression of an appropriately sized HA-HUM-7 and by immunostaining HA-HUM-7 is diffusely localized. I used the C-terminal half of HUM-7 to screen a yeast 2-hybrid library and a “bookshelf” of 26 known IAC components, but no interactors were identified. HUM-6 was identified previously by Hiroshi Qadota and found to interact with multiple IAC components. HUM-6 is a class VII unconventional myosin. Two deletion alleles and 2 nonsense alleles of hum-6 each show a unique defect at the MCB—aggregates of IAC proteins as a row of dots along the MCB arc. My overall hypothesis is that these class IX and class VII myosins are molecular trucks that deliver proteins to sites of IAC assembly in muscle.
Table of Contents
Abstract……………………………………………………………………………………………………………………………………..1
Introduction and Preliminary Findings……………………………………………………………………………………….2
Results and Discussion……………………………………………………………………………………………………………….9
Ongoing Works and Future Directions……………………………………………………………………………………..11
Methodology……………………………………………………………………………………………………………………………14
References……………………………………………………………………………………………………………………………….18
Figure 1. Schematic representation of predicted domains in HUM-7……….………………………………..4
Figure 2. Mutations in 2 unconventional myosins show MCB defects………………………………………..5
Figure 3. The localization of PIX-1 is not affected in hum-7 mutants..………………………………………..5
Figure 4. Full-length deletion of HUM-7 results in reduced locomotion………….………….……………..7
Figure 5. missense mutations in the HUM-7 myosin domain result in reduced locomotion…..…..7
Figure 6. Full-length deletion of HUM-7 shows no accumulation of PAT-6 at MCB………..…………..7
Figure 7. Schematic representation of domain in HUM-6…………………………………………………………..8
Figure 8. Western blot and Immunostaining of hum-7(syb11210)……………………………………………..9
Table 1. Bookshelf of IAC component screened for interaction with HUM-7……………………………11
Figure 9. Immunostaining of hum-7(syb11204) [R481Q] mutation with anti-PAT-6 …………………12
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