Proteins of the human placental microvillar cytoskeleton. α-Actinin

Abstract

The human placental syncytiotrophoblast microvilli are supported by an underlying cytoskeleton consisting mainly of actin microfilaments. The major proteins associated with the actin have Mr values of 105 000, 80 000 and 68 000. The 105 000-Mr protein is recognized by an antibody preparation raised to purified chicken gizzard alpha-actinin. Electron microscopy has shown that the human placental protein has dimensions similar to those reported for muscle alpha-actinin. About half of the placental microvillar alpha-actinin is released from the cytoskeleton in the presence of Ca2+. This effect occurs at concentrations of Ca2+ greater than 0.3 muM and has been used as the basis of a method for the purification of the placental alpha-actinin. This sensitivity to Ca2+ is not affected by trifluoperazine and is therefore likely to be a property of the alpha-actinin as such rather than being mediated via calmodulin.

Cytovillin and other microvillar proteins of human choriocarcinoma cells

“…Besides actin, a-actinin was found to be one of the major components of the JEG-3 microvilli. Moreover, a-actinin has been found in placental [4] and in MAT-C1 microvilli [28], but in the intestinal brush border a-actinin is located in the terminal web region beneath the microvillar array 1351. The major apparent difference between the JEG-3 and the MAT-C1 microvilli is the absence of tropomyosin in the JEG-3 microvilli, although the protein was present in the rest of the JEG-3 cell.…”

“…also the major microvillar proteins of human placenta [4] and of MAT-C1 mammary adenocarcinoma cells [28]. A third major band had an M, value of 35,000.…”

“…However, these microvilli are more dynamic with a less organized structure than their intestinal counterparts [ 31. The placental microvilli contain significant quantities of a-actinin associated with the cytoskeletal core [4], but are apparently devoid of villin and the 1 10 kd protein [ 5 ] . A group of placental microvillar proteins of M, 72,000,69,000, 38,000, 36,000, and 32,000 has been reported to be associated at high concentrations of free calcium with the cytoskeleton and the lipids [6].…”

Ezrin oligomers are major cytoskeletal components of placental microvilli: a proposal for their involvement in cortical morphogenesis.

“…(Edwards and Booth, 1987;Booth and Vanderpuye, 1983;Truman and Ford, 1986;Vanderpuye et al, 1986). Among the most abundant polypeptide components are several members of the annexin family of proteins (Edwards and Booth, 1987), actin, a 70-kD doublet, ezrin, and et-actinin (Vanderpuye et al, 1986). Treatment of the microvilli with RIPA buffer, which is often used to solubilize cells for immunoprecipitation studies, extracted surprisingly little of the actin, about half of the ezrin, and little of the ct-actinin.…”

“…Similar concentrations of ezrin are seen in both fractions, indicating that ezrin accounts for most or all of the major 81-kD band seen in total microvillus proteins. (Edwards and Booth, 1987;Booth and Vanderpuye, 1983;Truman and Ford, 1986;Vanderpuye et al, 1986). Among the most abundant polypeptide components are several members of the annexin family of proteins (Edwards and Booth, 1987), actin, a 70-kD doublet, ezrin, and et-actinin (Vanderpuye et al, 1986).…”