Lattice-like array particles on Xenopus oocyte plasma membrane.

Plasma membrane from Xenopus laevis oocytes has been used as a model system to study membrane structure and particle components, including native and exogenously expressed proteins. Previous studies by electron microscopy (EM) and atomic force microscopy (AFM) compared intramembrane particles (IMPs) on uninjected oocyte membranes to oocytes expressing proteins of interest. These studies observed randomly distributed IMPs on the surface of the oocyte plasma membrane. In this paper, we introduce a novel technique to isolate oocyte membranes by bursting the oocyte and depositing its membrane on a flat mica substrate. The flat surface membrane preparation allows high-resolution AFM images to beobtained, revealing a novel structure of densely packed particles. These particles exhibit a regular, repeating pattern of a lattice-like array with orderly packing and are thus termed "lattice-like array particles" (LAPs). The LAPs are orderly yet imperfectly packed, are located in depressed pools, occur with a low frequency on the oocyte membrane surface, and have not previously been seen using other isolation and imaging methods. Histogram analysis of the center-to-center distance between LAPs suggest their size to be about 44 nm in diameter, considerably larger than other reported size estimates of IMPs. These results indicate that LAPs represent a novel membrane particle organization, which merits further study.