Reevaluation of the hydrophobic nature of the 110‐kD calmodulin‐, actin‐, and membrane‐binding protein of the intestinal microvillus

A complex of calmodulin (CM) and the 110‐kD (110K) subunit composes the helical array of cross‐bridges linking the microvillus actin filament bundle with the membrane. The hydrophobic properties of the 110K protein, assessed by the detergent phase partitioning assay [Bordier C: J Biol Chem 256:1604, 1981], are highly dependent on the solution conditions used in its isolation. The ATP‐dissociable 110K‐CM complex [Howe and Mooseker: J Cell Biol 97:974, 1983] exhibits hydrophilic characteristics in this assay. In contrast, the 110K subunit extracted from brush borders by Triton X‐100, sodium dodecyl sulfate, and sodium pyrophosphate (detergent‐treated 110K) [Glenney JR, Glenney P: Cell 37:743, 1984] behaves as a hydrophobic protein. However, because the soluble hydrophilic 110K‐CM can be rendered hydrophobic by treating the complex with the same detergent and salt conditions used in the preparation of detergent‐treated 110K, the properties of detergent‐treated 110K seem likely to be an effect of the solution conditions on its native conformation, sedimentability, or exposure of binding domains. In addition, the detergent‐treated 110K is devoid of calmodulin and no longer exhibits the actin‐binding activity characteristic of the ATP‐dissociable 110K‐CM and of the intact complex in situ. With two partially purified preparations of the 110K subunit exhibiting such dramatically distinct properties, it seems premature to define the nature of the 110K subunit's association with the membrane at this time.