Labeling and isolation of plasma membranes from corn leaf protoplasts.

A plasma membrane-enriched fraction has been isolated from corn leaf mesophyll protoplasts and its identity confirmed with the aid of an external label, diazotized [(125)I]iodosulfanilic acid. Gentle cell disruption enabled internal organelles to be maintained intact and thus facilitated separation from the plasma membrane. The plasma membrane-enriched fraction was devoid of chloroplast or mitochondrial markers, whereas markers for the endoplasmic reticulum and golgi indicated minimal contamination. The highly enriched plasma membrane fraction contained a Mg(2+)-dependent, K(+)-stimulated ATPase with a pH optimum near neutrality. The position of the membranes on sucrose density gradients indicates that the plasma membranes have characteristics similar to other plasma membrane fractions.