Chemistry Reference
In-Depth Information
knock-down experiments using anti-sense oligonucleotides, UNC-45a antisense se-
verely reduced myoblast proliferation and fusion while UNC-45b antisense results
in significant loss of sarcomere organization (Price et al.
2002
). Reduction of UNC-
45b mRNA did not affect the level of skeletal myosin heavy chain, whereas lower-
ing of UNC-45a levels by antisense did (Price et al.
2002
). Low levels of UNC-45a
result in reduction of cell proliferation and differentiation which decreases the ex-
pression of sarcomeric myosin. The expression of UNC-45b, however, starts at the
fusion stage; therefore it may not affect myosin synthesis but rather its organization
into thick filaments. Thus the differential expression of the two UNC-45 isoforms
separates myosin synthesis from its organization into myofilaments. Interestingly,
UNC-45a has been reported to have higher apparent affinity and greater folding ca-
pability of smooth muscle myosin motor domain, compared to UNC-45b (Liu et al.
2008
). In addition, UNC-45A prefers to bind with Hsp90ʲ
in vitro
and it was shown
that UNC-45A is essential for Hsp90ʲ but not Hsp90α to assume normal cellular
distribution in HeLa cell via siRNA knockdown experiments (Chadli et al.
2008
).
Two isoforms of UNC-45A were detected in several breast carcinomas: a
929-amino-acid protein isoform and a 944-amino-acid protein isoform, which dif-
fer by an N-terminal proline-rich 15-amino-acid sequence (Guo et al.
2011
). The
protein level of the 929-amino-acid protein was found to be 3-fold higher due the
944-amino acid protein being degraded at a 5-fold higher rate that the former by
the ubiquitin-proteosome system (Guo et al.
2011
). Immunoprecipitation experi-
ments showed that both UNC-45A isoforms could interact with non-muscle myosin
IIA, non-muscle myosin IIB, and Hsp90ʲ, suggesting that both UNC-45A isoforms
are capable to play functional roles in cell motility (Guo et al.
2011
). Further ex-
perimental evidence that proved that the two isoforms of UNC-45 are functionally
divergent was obtained from studies using zebrafish (Comyn and Pilgrim
2012
).
The authors found out that the singly homologous zygotic mutant
unc45b
-/-
and
the doubly homologous mutant u
nc45b
-/-;
u
nc45a
-/-
displayed identical defects in
cardiac and skeletal muscle and jaw formation Comyn and Pilgrim
2012
). They
therefore concluded that unc-45a did not appear to play an important role in muscle
development and that the two gene paralogs are functionally divergent.
To elucidate the importance of muscle-specific UNC-45b's function
in vivo
, stud-
ies were carried out on three UNC-45b recessive loss-of-function lines in C3H and
C57BL/6 inbred mouse strains (Chen et al.
2012
). These mutations caused arrest of
cardiac morphogenesis at the formation of right heart structures and failure of con-
tractile function (Chen et al.
2012
). A novel outcome of these experiments was the
finding that UNC-45b was essential for sufficient accumulation and function of the
cysteine-zinc finger protein GATA4 (an important transcription factor during devel-
opment) in mouse embryonic cardiac morphogenesis (Chen et al.
2012
). Pull-down
experiments confirmed a direct physical interaction between UNC-45b and GATA4
(Chen et al.
2012
). Therefore, the heart-specific UNC-45b isoform functions as a
molecular chaperone mediating contractile function of the sarcomere and gene ex-
pression in cardiac development. Recently, UNC-45B has been reported to be in-
volved in lens development and pathogenesis of autosomal dominant juvenile cata-
ract in humans (Hansen et al.
2014
). It is hypothesized that developmental cataract
Search WWH ::
Custom Search