Smooth Myocytes - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 380

8.1.1 Actin

Two classes and 3 main groups of actin isoforms exist: class- 1 , non-muscle actin,
which encompasses

β and γ groups, and class- 2 , muscle actin, which corresponds

to α actins are constituents of the contractile apparatus.

Smooth myocytes possess class-2

group. In myocytes, α

α 2- and α 3- (or γ 2)-actins, in addition to

class-1 actins. Isoforms coexist in most cell types as components of

microfilaments of the cytoskeleton that support signal transduction, cell adaptation
to mechanical loads, intracellular transport, and cell motility, among other functions
(Vol. 1 - Chap. 6. Cell Cytoskeleton).

β and γ

8.1.2 Tropomyosin

Tropomyosin polymers (TpM1-TpM4) are actin-binding proteins that regulate
the binding of myosin, as they block access to myosin-binding sites on actin.
Tropomyosins reside along each of the 2 grooves of actin filaments to yield
structural stability and modulate filament function.
Many isoforms are produced by alternative promoters of 4 genes (TPM1-TPM4)
as well as RNA processing. Many of these isoforms have a tissue- and filament-
specific distribution.

8.1.3 Myosin

Myosin are adenosine triphosphatases that constitute a superfamily (Vol. 1 -
Chap. 6. Cell Cytoskeleton). Non-conventional myosins, such as myosin-5 and -
6 that travel toward the barbed (or

) ends of actin filaments,
respectively, are devoted to cargo transfer within the cell. Conventional myosin, i.e.,
myosin-2, contains 2 heavy and 2 pairs of light chains. Myosin heavy chains contain
N-terminal heads and C-terminal tails that hold the 2 heavy chains together in a
coiled-coil manner. Two myosin regulatory light chains per heavy chain N-terminus
bind the region (neck) between the head and tail.
Four known isoforms of smooth muscle myosin heavy chain (smMHC) are
formed by 2 independent, alternative splices of the transcript of a single gene
(smMHC1a-smMHC1b and smMHC2a-smMHC2b). 4 Isoforms of the mMHC i b
type contain an additional 7 amino acids that double Mg ATPase activity as well as
the displacement rate on actin filaments [ 705 ].

+ ) and pointed (or −

4 Alternative splicing in the region that encodes the heavy chain tail produces smMHC1 and
smMHC2 isoforms. Alternative splicing in the region that encodes the 25-50-kDa junction
generates the smMHCa and smMHCb isoforms. The proportion of smMHCa and smMHCb differs
in tonic and phasic smooth muscles.