Alpha-actinin-4

α-actinin-4 (Actn4) is a member of the spectrin-family of actin crosslinking proteins and is the most recently identified isoform (Dixson et al. 2003; Otey and Carpen 2004; Virel and Backman. 2004; Honda et al. 1998). Actn4 exists as an anti-parallel homodimer. The N-terminus consists of two highly conserved tandem calponin-homology domains (CH1, CH2) which are required for F-actin binding. The CH domains are connected to a series of four spectrin-like coiled-coil repeats which make up the rod domain that facilitates dimerization. Once regarded as simply spacers for actin bundle formation, these spectrin repeats have a newfound role as a docking site or scaffold for many signaling proteins (Djinovic-Carugo et al. 2002). The repeats are followed by a calmodulin-like domain that is composed of two EF hand domains. Aside from its role as an actin crosslinking protein, Actn4 serves to anchor integral membrane proteins or scaffolding proteins for signaling molecules to the actin cytoskeleton. Like Actn1, Actn4 is a calcium sensitive isoform (Burridge and Feramisco. 1981). It has 80% nucleotide and 86.7% amino acid similarity to Actn1. Despite this similarity, the roles of Actn4 and Actn1 are not functionally redundant as demonstrated by Kos et al. (2003).

As Actn4 exists as an anti-parallel homodimer, the calmodulin-like domain of one molecule is in very close proximity to the actin-binding domain of the neighboring molecule. This suggests that Actn4’s ability to bind actin involves calcium binding to the EF hands (Burridge and Feramisco. 1981).

Actn4 binds to F-actin via its N-terminal actin-binding domain.

Actn4 interacts with cytoskeletal scaffolding and signaling proteins. The following interactions have been demonstrated to be specific for the Actn4 isoform: Actn4 interacts with the hemidesmosome transmembrane protein BP180 (Gonzalez et al. 2001). The PDZ-LIM protein Elfin/CLP-36 interacts with Actn4 at the stress fibers of colonic epithelium via binding of the PDZ domain to the spectrin repeats (Vallenius et al. 2000). PAI-1 (plasminogen activator inhibitor type 1), a protein with roles in inflammation, tumor invasion and metastasis, interacts specifically with Actn4 (Magdolen et al. 2004). Actn4 also anchors the enzyme iNOS (inducible nitric oxide synthase) to the actin cytoskeleton in macrophages (Daniliuc et al. 2003). SPA-1 is involved in the regulation of T cell activation in response to antigens through the control of Rap1 GTPase signaling, and is localized to the immunuological synapse via interaction with the Actn actin-binding domain (Harazaki et al. 2004).

Actn4 is also identified as a component of several multi-protein complexes. It is suggested that BERP, a novel RING finger protein, may anchor class V myosins to particular cell domains via its interaction with Actn4 (El-Husseini et al. 2000). Furthermore, BERP is part of the CART complex for cytoskeleton-associated recycling or transport consisting of hrs/Actn4/BERP/myosin V (Yan et al. 2005). RN-tre is a Rab5-GAP/effector that is involved in the formation of circular ruffles at the plasma membrane where it establishes a three-pronged connection with Rab5, F-actin and Actn4 (Lanzetti et al. 2004). The tight junction protein MAGI-1 binds to the C terminus of Actn4 via its fifth PDZ domain, while also binding synaptopodin in the polarized epithelial cells of the kidney (Patrie et al. 2002). The transmembrane synaptic adhesion molecule densin-180 forms a ternary complex with Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and Actn4 in the neural PSD and kidney podocytes. Densin-180 binds to Actn4 via its PDZ domain (Walikonis et al. 2001; Ahola et al. 2003). Another ternary complex is the NHE3/E3KARP/Actn4 in which E3KARP binds to the actin binding domain of Actn4 (maximal at 1 μM [Ca²⁺]i) to regulate Na/H exchanger 3 (Kim et al. 2002).

Actn4 has also been reported to interact with nuclear proteins. Actn4 interacts with NF-Y to recruit chromatin-remodeling complexes or to direct NF-Y/Actn4-targeted genes to the nuclear matrix and active transcriptional complexes (Poch et al. 2004). DNaseY is shown to interact with Actn4 and this interaction significantly enhances its endonuclease activity (Liu et al. 2004). Even more intriguing is the presence of Actn4 in the extracellular matrix as a result of cell movement. It is cleaved by monocyte-secreted urokinase to produce a 31 kDa amino-terminal fragment of Actn4 termed mactinin. Mactinin is shown to be a promoter of monocyte/macrophage maturation (Luikart et al. 1999; Masri et al. 1999; Luikart et al. 2002; Luikart et al. 2003).

No information is available yet.

Actn4 is found at stress fibers, microfilament bundles, and sites of cell contact or adhesion. Its localization may differ somewhat from that of Actn1, the other calcium-sensitive isoform. It has also been reported that Actn4 may translocate to the nucleus in some cells.

Actn4 expression is widespread (Honda et al. 1998).

Overexpression of cytoplasmic Actn4 is associated with increased cell motility and a decrease in Actn1 at sites of adhesion. Inactivation and translocation of Actn4 to the nucleus is linked to decreased metastatic potential.

A mutation in ACTN4 has been implicated in Focal Segmental Glomerulosclerosis. A possible alternative splice variant may be associated with small-cell lung carcinoma.

A possible alternative splice variant may be associated with small cell lung carcinoma.

Mouse monoclonal antibody (mAb) to chicken gizzard actinin, clone JLN20 (ICN, InnoGenex, BioGenex, Oncogene Research Products).

Mouse mAb to human actinin, clone AT 6/172 (Research Diagnostics Inc, Immune Systems Ltd).

Mouse mAb to quail smooth muscle actinin, 1E12 (Developmental Studies Hybridoma Bank at the University of Iowa).

Mouse mAb to chicken gizzard, clone CB11 (ICN, Affiniti).

Rabbit polyclonal Ab to chicken gizzard (ICN).

Mouse mAb to human red blood cell ghosts, clone RBC2/1B6 (Novocastra).

Mouse mAb to smooth muscle actinin, clone 1A4 (Diagnostic BioSystems).

Goat polyclonal Ab to aminoterminus of human ACTN1, N-19 (Santa Cruz Biotechnology).

Goat polyclonal Ab to human ACTN1 carboxyterminus, C-20 (Santa Cruz Biotechnology).