2 EYE GUANYLYL CYCLASES ARE EXPRESSED IN THE SAME PHOTORECEPTOR CELLSAND FORM HOMOMERS IN PREFERENCE TO HETEROMERS

We recently described two eye guanylyl cyclases (GC-E and GC-F) that c ontain an apparent extracellular domain potentially capable of binding ligands (Yang, R.-B., Foster, D, C., Garbers, D. L., and Fulle, H.-J, (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 602-606). Here, Northern a nd Western analyses showed that both cyclases are expressed in the ret ina and enriched in photoreceptor outer segments. By the use of specif ic GC-E and GC-F antibodies coupled to different sized gold particles both cyclases were colocalized within the same photoreceptor cells rai sing the possibility of homomeric and/or heteromeric interactions. A p oint mutant of GC-E (D878A) was constructed and expressed; it containe d no detectable cyclase activity but acted in a dominant negative fash ion to abolish the activity of native GC-E and GC-F in coexpression st udies. These results suggested that GC-E and GC-F could form either ho momers or heteromers, at least when overexpressed in COS-7 cells. Immu noprecipitation with GC-E and GC-F antibody followed by Western analys is confirmed that both homomers and heteromers could be formed. Howeve r, similar experiments using retina or outer segments revealed that a vast majority of GC-E and GC-F were precipitated as homomers in the ey e. Therefore, like other members of the membrane guanylyl cyclase subf amily, GC-E and GC-F appear to preferentially form homomers.