Regular structures in unit membranes. III. Further observations on the particulate component of the suckling rat ileum endocytic membrane complex

Further morphological observations on the particulate components decorating the lumenal surfaces of membranes of the endocytic complex of the epithelial cells of the suckling rat ileum are presented. The particles each measure approximately 7.5 nm across and give the appearance of the capital letter H in frontal view. They consist of the enzyme n-acetyl-beta-glucosaminidase (NAG). They are arranged in rows called "decorated strips" with the symmetrical lateral bars in register and spaced approximately 14.5 nm apart. Decorated strips lie side-by-side in the external (lumenal) surface of the membrane. They are parallel and sometimes spaced approximately 14.5 nm apart making an orthogonal lattice. The lateral spacing between the decorated strips under certain conditions is reduced and sometimes there is shear between the adjacent ones. Occasionally, shear is present within the decorated strips themselves, with slight displacement of the two sides of each H-shaped particle. A purified preparation of these membranes has been studied by electron microscopy using thin sectioning, negative stain, Markham translation and optical diffraction computer image reconstruction methods. The individual particles comprising the array can be seen in the membrane surface in profile view when dried in a pool of negative stain. They appear either triangular or diamond-shaped in such views. If triangular, they appear to consist of three domains at the corners of an equilateral triangle. One side of each triangular figure is parallel to the membrane surface but separated from it by a dense band of negative stain approximately 2 nm thick that runs along the surface of the membrane. Sometimes a fourth symmetrical domain is visible within this dense band, giving a diamond-shaped figure. This fourth domain connects the particle to the membrane. Thus, each H-shaped particle is a double structure, with each half in profile view appearing as a diamond figure of four symmetrical domains. Each H-shaped particle is believed to consist of either two or four molecules of NAG.