Cubic membrane structure in amoeba (Chaos carolinensis) mitochondria determined by electron microscopic tomography

Cubic membranes occur in a variety of membrane-bound organelles in many cel l types. By transmission electron microscopy (TEM) these membrane systems a ppear to consist of highly curved periodic surfaces that fit mathematical m odels analogous to those used to describe lipidic cubic phases. For the fir st time, a naturally occurring cubic membrane system has been reconstructed in three dimensions by electron microscopic tomography, and its periodicit y directly characterized. Double-tilt tomographic reconstruction of mitocho ndria in the amoeba, Chaos carolinensis, confirms that their cristae (inner membrane infoldings) have the cubic structure suggested by modeling: studi es based on thin-section TEM images. Analysis of the membrane surfaces in t he reconstruction reveals the connectivity of the internal compartments wit hin the mitochondria. In the cubic regions, the matrix is highly condensed and confined to a continuous, small space between adjacent cristal membrane s. The cristae form large, undulating cisternae that communicate with the p eripheral (inner membrane) compartment through narrow tubular segments as s een in other types of mitochondria. The cubic periodicity of these mitochon drial membranes provides an ideal specimen for measuring geometrical distor tions in biological electron tomography. It may also prove to be a useful m odel system for studies of the correlation of cristae-matrix organization w ith mitochondrial activity. (C) 1999 Academic Press.