HISTIDINE TAGGING BOTH ALLOWS CONVENIENT SINGLE-STEP PURIFICATION OF BOVINE RHODOPSIN AND EXERTS IONIC STRENGTH-DEPENDENT EFFECTS ON ITS PHOTOCHEMISTRY

For rapid single-step purification of recombinant rhodopsin, a baculov irus expression vector was constructed containing the bovine opsin cod ing sequence extended at the 3'-end by a short sequence encoding six h istidine residues, Recombinant baculovirus-infected Spodoptera frugipe rda cells produce bovine opsin carrying a C terminal histidine tag (v- opshis(6x)). The presence of this tag was confirmed by immunoblot anal ysis. Incubation with 11-cis-retinal produced a photosensitive pigment (v-Rhohis(6x)) at a level of 15-20 pmol/10(6) cells. The histidine ta g was exploited to purify v-rthohis(6x) via immobilized metal affinity chromatography. Optimized immobilized metal affinity chromatography y ielded a binding capacity of greater than or equal to 35 nmol of v-Rho his(6x) per ml of resin and purification factors up to 500. Best sampl es were at least 85% pure, with an average age purity of 70% (A(280) n m/A(500) nm = 2.5 +/- 0.4, n = 7). Remaining contamination was largely removed upon reconstitution into lipids, yielding rhodopsin proteolip osomes with a purity over 95%. Spectral analysis of v-Rhohis(6x) showe d a small but significant red shift (501 +/- 1 nm) compared to wild ty pe rhodopsin (498 +/- 1 nm). The pK(a) of the Meta I <-> Meta II equil ibrium in v-Rhohis(6x) is down-shifted from 7.3 to 6.4 resulting in a significant shift at pH 6.5 toward the Meta I photointermediate. Both effects are reversed upon increasing the ionic strength. FT-IR analysi s of the Rho --> Meta II transition shows that the corresponding struc tural changes are identical in wild type and v-Rhohis(6x).