Cell-to-substrate contacts in an adhesion-defective mutant of Balb/c3T3 cells

The cell-to-substrate contacts of the adhesion-defective mutant, AD6, have been examined by interference reflexion microscopy and compared with those of the wild-type Balb/c3T3 cell. The 2 cell-types differed in their ability to produce focal contacts with the substrate (10-15 nm separation distance). Only 10% of AD6 versus 92% of Balb/c3T3 cells formed these contacts. When present in AD6 the focal contacts were smaller and fewer in number per cell than in Balb/c3T3. Close contact with the substrate (approx. 30 nm separation distance) was formed by both cell types. The absence of the stronger focal contacts accounts for the reduced substrate adhesion of the mutant cells and for other phenotypic characteristics such as reduced spreading, rounded shape, altered pattern of movement, and absence of stress fibres. The reduced adhesion of the mutant has been attributed to a change in the pattern of glycoproteins exposed at the cell surface, which results from a defect in glycosylation. This suggests that normal glycosylation and correct exposure of one or more cell surface components are required for the formation of a specific substrate adhesion, the focal contact, and offers the possibility of identifying individual surface components involved in formation of this adhesion. Attention is drawn to zero-order minima originating from cell thickness and not cell-to-substrate separation distance in the interference reflexion image.