Abstract:
Myosins are actin-based molecular motors that may have specialized trafficking
and contractile functions in cytoskeletal compartments that lack microtubules.
The postsynaptic excitatory synapse is one such specialization, yet little is known
about the spatial organization of myosin motor proteins in the mature brain. We
used a proteomics approach to determine if class II and class V myosin isoforms
are associated with Triton X-100-resistant membranes isolated from mouse
forebrain. Two nonmuscle myosin isoforms (II-B and Va), were identified as
components of lipid raft fractions that also contained typical membrane skeletal
proteins such as non-erythrocyte spectrins, actin, alpha-actinin-2 and tubulin
subunits. Other raft-associated proteins included lipid raft markers, proteins
involved in cell adhesion and membrane dynamics, receptors and channels
including glutamate receptor subunits, scaffolding and regulatory proteins.
Myosin II-B and Va were also present in standard postsynaptic density (PSD)
fractions, however retention of myosin II-B was strongly influenced by ATP
status. If homogenates were supplemented with ATP, myosin II-B could be
extracted from PSD I whereas myosin Va and other postsynaptic proteins were
resistant to extraction. In summary, both myosin isoforms are components of a
raft-associated membrane skeleton and are likely detected in standard PSD
fractions as a result of their intrinsic ability to form actomyosin. Myosin IIB,
however, is loosely or more peripherally associated with the PSD than myosin
Va, which appears to be a core PSD protein.
