Analysis of nuclear protein complexes comprising α-actinin-4 by 2D-electrophoresis and mass spectrometry

Actin-binding protein α-actinin-4 is a member of the spectrin superfamily; it is located in both the cytoplasm and nucleus. However, nuclear functions of α-actinin-4 are still not clear. In this study, we analyzed the composition of nuclear protein complexes associated with α-actinin-4 in A431 cells. Using 2D electrophoresis, we found that about 50 different proteins may be associated with nuclear α-actinin-4. Major proteins of these complexes were analyzed by mass spectrometry and the following proteins have been identified: β-actin, α- and β-tubulins, ribonucleoprotein A2/B1 (regulates splicing and is associated with β-actin), peroxiredoxin-1 (involved in oxidative stress), and glycolytic enzyme D-3-phosphoglycerate dehydrogenase. The detection of these proteins in nuclear complexes with α-actinin-4 may suggest that α-actinin-4 is implicated in transcription and splicing. The presence of β-actin in these complexes was confirmed by tandem mass spectrometry (MALDI-TOF-TOF). The immunoprecipitation of nuclear proteins with antibodies against α-tubulin proved the association of α-actinin-4 with α-tubulin in the protein complex. Nuclear α-actinin-4 consists of a 105-kDa, full-size isoform and two truncated isoforms of 65 and 75 kDa; only truncated isoforms have been found in nuclear complexes with α-tubulin. These data suggest that α-actinin-4 is associated with a number of various nuclear protein complexes that may be able to carry out different functions in the cell nucleus.