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

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Abstract
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 .apprx. 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 .apprx. 14.5 nm apart. Decorated strips lie side-by-side in the external (lumenal) surface of the membrane. They are parallel and sometimes spaced .apprx. 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 2 sides of each H-shaped particle. A purified preparation of these membranes was studied by EM using thin sectioning, negative stain, Markham translation and optical diffraction computer image reconstruction methods. The individual particles comprising the array were 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 3 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 .apprx. 2 nm thick that runs along the surface of the membrane. Sometimes a 4th symmetrical domain is visible within this dense band, giving a diamond-shaped figure. This 4th 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 4 symmetrical domains. Each H-shaped particle is believed to consist of either 2 or 4 molecules of NAG.