A chondroitin sulfate keratan sulfate proteoglycan, PG-1000, forms complexes which are concentrated in the reticular laminae of electric organ basement membranes

Previously, we identified PG-1000 as part of a disulfide-linked complex of two large proteoglycans (PG-1000 and the beta component) and three smaller proteins purified from the extracellular matrix of elasmobranch electric organ (Iwata and Carlson, 1991, J. Biol. Chem. 266: 323-333). PG-1000 is a chondroitin sulfate/keratan sulfate proteoglycan with a molecular mass of about 1.2 x 16(6) daltons. When visualized in the electron microscope, PG-1000 has the typical ''bottle-brush'' appearance expected for a proteoglycan with an average total length of about 345 nm and about 20 chains of similar to 110 nm (Carlson and Wight, 1987, J. Cell Biol. 105: 3075-3086). Using immunocytochemical methods, we now demonstrate that PC-1000 is a component of the interstitial extracellular matrix of the electric organ. PC-1000 immunoreactivity is found throughout the interstitial matrix, but it is highly concentrated in that region of the matrix immediately adjacent to the basal lamina, the reticular lamina. The reticular and basal laminae together form the basement-membrane. PC-1000 immunoreactivity is especially apparent on basal laminae that surround nerve fibers and nerve terminals. When the disulfide-linked PC-1000 complexes are purified and examined in the electron microscope following rotary shadowing, they appear as bottle-brush structures which are often attached at a central region and radiate like spokes of a wheel. These aggregates contain two to six proteoglycan monomers. We hypothesize that the PG-1000 complexes are disulfide-stabilized parts of an extended network of linked proteoglycans in the reticular lamina.