INTERACTION OF SURFACTANT PROTEIN-A WITH CELLULAR MYOSIN

The goal of the current investigation was to characterize, purify, and identify the proteins that bind surfactant protein A (SP-A). Several polypeptides were purified by SP-A-affinity chromatography, and the 20 0 kD major polypeptide that reacted with SP-A on ligand blots was puri fied further by preparative SDS-PAGE, Protein sequencing of proteolyti cally derived subfragments of this polypeptide gave sequences that cor responded completely with nonmuscle (cellular) myosin heavy chain. The 200 kD polypeptide was then found to be immunoreactive with antibodie s against cellular myosin. A smaller polypeptide of 135 kD also binds SP-A and appears to be a proteolytic fragment of the 200 kD peptide. T he ability of SP-A to bind myosin was confirmed in a microtiter well a ssay and was found to be concentration dependent. We speculated that t he physiologic relevance of the interaction of SP-A with myosin might be to facilitate clearance of myosin from the alveolar subphase follow ing its release during lung injury. In support of this hypothesis, we found that there were detectable levels of myosin in lavage fluid and that SP-A could indeed enhance uptake and degradation of myosin by alv eolar macrophages.