Functional interrelation of nuclear actin with transcriptional active chromatin in the interphase nucleus was reliably established in numerous experiments, but the relationship between actin and transcriptional silent chromatin is still unclear. We examined localization area of the second meiotic division metaphase plate in ovulated mouse oocytes with the aim to study the possibility of actin-chromatin colocalization and uncovering the distribution patterns of different functional forms of actin near the metaphase chromosomes. Confocal microscopy and probes for actin that are distinguished from each other by the mechanism of actin binding (TRITC-phalloidin, fluorescent DNase-I, and antibodies against fragment of C-terminal and fragment of N-terminal domain of actin) were used for actin visualization. Despite the fact that TRITC-phalloidin could not detect F-actin in the area of metaphase plate, oocytes staining with antibody against fragment of the actin N-terminal domain demonstrates the presence near the metaphase chromosomes of some spindle-like structure composed of actin filaments. Among all used probes for actin, only the antibody against fragment of the C-terminal domain detected accurate actin colocalization with metaphase chromosomes. We conclude that this antibody labeled noncanonical form of the nuclear actin existing in long-term association with highly condensed chromatin. 1. Introduction Actin is an important participant of eukaryotic cell metabolism. Within the nucleus actin exists in several functional forms—monomeric (G-actin), actin oligomers, and short filaments [1]. The presence of canonical filamentous actin (F-actin) in the nucleus under normal conditions has not been authentically proved. Results of numerous studies carried out with the use of various somatic cell types have demonstrated involvement of nuclear actin to such basic nuclear processes as mRNA processing, nuclear export, and intranuclear transport [2, 3]. By the present time convincing evidence of actin association with functionally active chromatin was obtained: actin was identified as a part of all three RNA polymerases [4] and of chromatin remodeling complexes [5]. But at the same time interrelations between actin and transcriptional silent highly condensed chromatin (or chromosomes) are still unclear. The functional role of actin in nuclear metabolism during gametogenesis and in initial stages of embryo development today is poorly understood. Historically the main part of data on cellular biology of nuclear actin has been received in experiments designed with the
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