Molecular basis of cytoskeletal regulation involving alpha-actinin-4

The actin cytoskeleton plays a fundamental role in cellular processes such as maintenance of cell morphology, adhesion, division, phagocytosis, and as a key part of the contractile apparatus. alpha-actinin dynamically cross-link actin filaments to determine mechanical properties of the actin filament network. alpha-Actinin-4, a non-muscle alpha-actinin isoform, localises to actively moving structures and is up-regulated upon enhanced cell movement. alpha-actinin-4 is particularly significant as it is suggested to be linked with the metastatic potential and invasiveness of human cancers and has been observed to suppress tumourgenicity of human neuroblastoma cells. It is likely that alpha-actinin-4 may act as an adaptor between cell-signalling proteins and the cytoskeleton and may providea mechanism for regulation of cytoskeletal remodelling.

Figure 1. Solution structure of the third spectrin repeat of alpha-actinin-4. This domain interacts with the PH domain of Tec, linking alpha-actinin-4 to Tecmediated signalling.

The alpha-actinins belong to the spectrin superfamily and contain four spectrin repeats. Spectrin repeats function
as a structural motif for the spatial coordination of cytoskeletal interactions or as a protein-recognition motif for
a number of regulatory proteins. We are interested in characterising the protein-protein interactions mediated
by alpha-actinin-4 in order to understand the molecular basis of cytoskeletal regulation.
We are presently using a combination of structural biology (using NMR techniques) and molecular biology to
investigate the interaction between the third spectrin repeat of alpha-actinin-4 and the PH domain of the Tec
tyrosine kinase. A similar approach will be taken to characterise the structures of domains involved in and
interactions mediated by alpha-actinin-4 and the following proteins:

1. CLP-36. CLP-36 is a PDZ-LIM protein that is an adaptor between cell signalling pathways and the cytoskeleton. It interacts with Ser/Thr kinases through its LIM domain and to alpha-actinin-4 through its PDZ domain.
2. BERP (brain-expressed RING finger protein). BERP belongs to the RBCC (RING B-box coiled-coil) family of proteins, which appear to play key roles in regulating gene expression and cell proliferation.