Isoelectric focusing of plant cell membranes

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Abstract
An isoelectric focusing method is described that discriminates plant cell organelle populations on the basis of surface charge. The isoelectric points (pI values) of the Golgi apparatus, the mitochondria and putative plasma membrane from etiolated pea stem cells are reported. The pI of the pea Golgi apparatus is similar to that of the 35SO4-labeled membranes of developing Fucus embryos. The pI of Fucus 35SO4-labeled membranes depends on whether the membrane are being polarly transported to the growing tip or randomly transported to the entire periphery of the zygote. Those subject to polar tip transport have a significantly greater negative surface charge than those being randomly transported. The implication of this result toward an understanding of the localization mechanism is discussed. The method is also capable of subfractionating glucan synthetase II-containing membranes (putative plasma membrane) from peas. The pI of putative plasma membrane from a pea stem homogenate is similar to the pI of the plasma membrane of whole protoplasts of Catharanthus leaf cells. Isoelectric focusing apparently is a useful technique to discriminate membranes, and, thus, to provide new information and approaches to study cellular and developmental phenomena.