Microvillus 110K-calmodulin: effects of nucleotides on isolated cytoskeletons and the interaction of the purified complex with F-actin.

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Abstract
Microvilli isolated from intestinal epithelial cells contain a cytoskeletal Mr 110,000 polypeptide complexed with calmodulin (110K-CM) that is believed to link the microfilament core bundle laterally to the plasma membrane. Previous work has shown that physiological levels of ATP can partially solubilize the 110K-CM complex from isolated microvillus cytoskeletons or isolated microvilli. However, once extracted, the 110K-CM complex has been found to be difficult to maintain stably soluble in aqueous buffers. This is due to the presence of an endogenous ATPase (approximately 100 nmol Pi/min per mg at 37 degrees C) in microvillus cytoskeletal preparations that depletes the ATP with subsequent precipitation of 110K-CM. Addition of ATP to such precipitates resolubilizes 110K-CM. Inclusion of an ATP regenerating system in the solubilization of 110K-CM from cytoskeletons, or membrane-bound brush borders, increases the amount of 110K-CM solubilized. Solubilization of 110K-CM from microvillus cytoskeletons was found to require a divalent cation (Mg2+, Mn2+, or Co2+, but not Zn2+) and a nucleoside triphosphate (ATP, GTP, CTP, or ITP). ADP did not solubilize 110K-CM, but could partially inhibit ATP-dependent solubilization. Solubilized 110K was phosphorylated during extraction of microvillus cores with [gamma-32P]ATP, but this was unrelated to the solubilization of 110K-CM as the endogenous kinase was specific for ATP, whereas the solubilization was not. The 110K-CM was purified using ATP extraction of brush border cytoskeletons in the presence of an ATP regenerating system, gel filtration of the solubilized extract, an ATP depletion step to specifically precipitate 110K-CM with F-actin, and resolubilization followed by phosphocellulose chromatography. The purified complex was stably soluble in aqueous buffers both in the presence and absence of ATP. It bound almost quantitatively to F-actin in the absence of ATP, and showed nucleotide solubilization characteristics from F-actin similar to that found for solubilization of 110K-CM from microvillus cores. At low ATP levels, the binding to F-actin was increased in the presence of ADP. These results suggest that the purified complex has been isolated in a native form. The data confirm and extend the studies of Howe and Mooseker (1983, J. Cell Biol., 97:974-985) using a partially purified preparation of 110K-CM and further emphasize that 110K-CM is a stably water soluble complex and not an integral membrane protein.