CELL-CELL ADHESION PREVENTS MUTANT-CELLS LACKING MYOSIN-II FROM PENETRATING AGGREGATION STREAMS OF DICTYOSTELIUM

When a small number of fluorescently labeled myosin II mutant cells (m hcA(-)) are mixed with wild-type cells and development of the chimeras is observed by confocal microscopy, the mutant cells are localized to the edges of aggregation streams and mounds. Moreover, the mutant cel ls stick to wild-type cells and become distorted (Shelden and Knecht, 1995). Two independent adhesion mechanisms, Contact Sites A and Contac t Sites B, function during the aggregation stage and either one or bot h might be responsible for excluding the myosin II null cells. We have mixed mhcA(-) cells with cells in which the appearance of Contact Sit es B is delayed (strain TL72) as well as cells which lack Contact Site s A (strain GT10) and double mutants in which both adhesion mechanisms are affected (strain TL73). In all chimeras, the mhcA(-) cells were d istorted by interactions with the adhesion mutant cells, indicating th at it does not require significant adhesive interaction to distort the flaccid cortex of mhcA(-) cells. mhcA cells were excluded from stream s composed of cells lacking either Contact Sites A or Contact Sites B but mixed randomly with cells lacking both adhesion systems. By 10 hr of development, cells of strain TL73 acquire Contact Sites B adhesion. If cells of this strain were mixed with labeled mhcA(-) cells, allowe d to develop for 9 hr, and then dissociated before replating, the myos in II null cells were seen to be distorted and excluded from the reagg regates. Thus the exclusion of mhcA(-) cells from streams can be accom plished by either Contact Sites A or B. When chimeras of labeled TL73 and wild-type cells were made, the TL73 cells were found to be randoml y mixed into aggregation streams. This result indicates that adhesive sorting does not function during aggregation and so cannot account for the exclusion of mhcA(-) cells from streams. We hypothesize that the flaccid cortex of mhcA cells cannot generate sufficient protrusive for ce to break the contacts between adhered cells in aggregation streams but can enter streams where the cells are weakly adherent. (C) 1996 Ac ademic Press, Inc.