Nuclear Ac tin

Actin, actin-related proteins and numerous actin-binding proteins (including a nuclear-specific isoform of myosin I) are now known to be present in the nucleus, but their functions are emerging only slowly (Pederson and Aebi 2005). While there is no evidence yet for long actin filaments (F-actin), nuclear actin can form a multitude of dimers, short protofilaments and tubular, flat or branched oligomers. These nuclear actin polymers adopt a unique conformation that is recognized by specific antibodies. Since actins can also bind to two regions in the lamin A/C tail, they may be considered architectural partners of lamin filaments.

The presence of actin and nuclear myosin I (NMI) in the nucleus suggests a role for these motor proteins in nuclear functions. Although a direct participation in the nuclear matrix has remained uncertain, there have been hints for their interaction with nuclear RNAs and with proteins from hnRNP complexes (Percipalle et al. 2002), which, according to Kukalev et al. (2005), are essential for productive pol II transcription. Other studies demonstrate that antibodies against P-actin inhibit pol II transcription in a mammalian system for which actin was detected as a component of pre-initiation complexes and where it played a role in initiation (Hofmann et al. 2004). Since this inhibitory effect could be reproduced on naked DNA templates in vitro, the transcriptional role of actin does not seem to be restricted to chromatin remodelling complexes.

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