AUTOCRINE MOTILITY FACTOR-RECEPTOR IS A MARKER FOR A DISTINCT MEMBRANOUS TUBULAR ORGANELLE

Autocrine motility factor (AMF) is secreted by tumor cells and is capa ble of stimulating the motility of the secreting cells. In addition to being expressed on the cell surface, its receptor, AMF-R, is found wi thin a Triton X-100 extractable intracellular tubular compartment. AMF -R tubules can be distinguished by double immunofluorescence microscop y from endosomes labeled with the transferrin receptor, lysosomes labe led with LAMP-2, and the Golgi apparatus labeled with beta-COP. AMF-R can also be separated from a LAMP-2 containing lysosomal fraction by d ifferential centrifugation of MDCK cells and is found within a 100,000 g membrane pellet. By electron microscopic immunocytochemistry, AMF-R is localized predominantly to smooth vesicular and tubular membranous organelles as well as to a lesser extent to the plasma membrane and r ough endoplasmic reticulum. AMF-R tubules have a variable diameter of 50-250 nm and can acquire an elaborate branched morphology. By immunof luorescence microscopy, AMF-R tubules are clearly distinguished from t he calnexin labeled rough endoplasmic reticulum and AMF-R tubule expre ssion is stable to extended cycloheximide treatment. The AMF-R tubule is therefore not a biosynthetic subcompartment of the endoplasmic reti culum. The tubular morphology of the AMF-R tubule is modulated by both the actin and microtubule cytoskeletons. In a similar fashion to that described previously for the tubular lysosome and endoplasmic reticul um, the linear extension and peripheral cellular orientation of the AM F-R tubule are dependent on the integrity of the microtubule cytoskele ton. The AMF-R tubule may thus form part of a family of microtubule-as sociated tubular organelles.