Department of Biology, Lawrence University, Appleton, Wisconsin 54911, USA.
J Bacteriol. 2011 Oct;193(20):5658-67. doi: 10.1128/JB.05376-11. Epub 2011 Aug 12.
Integral membrane protein complexes consisting of proteins and small molecules that act as cofactors have important functions in all organisms. To form functional complexes, cofactor biosynthesis must be coordinated with the production of corresponding apoproteins. To examine this coordination, we study bacteriorhodopsin (BR), a light-induced proton pump in the halophilic archaeon Halobacterium salinarum. This complex consists of a retinal cofactor and bacterioopsin (BO), the BR apoprotein. To examine possible novel regulatory mechanisms linking BO and retinal biosynthesis, we deleted bop, the gene that encodes BO. bop deletion resulted in a dramatic increase of bacterioruberins, carotenoid molecules that share biosynthetic precursors with retinal. Additional studies revealed that bacterioruberins accumulate in the absence of BO regardless of the presence of retinal or BR, suggesting that BO inhibits bacterioruberin biosynthesis to increase the availability of carotenoid precursors for retinal biosynthesis. To further examine this potential regulatory mechanism, we characterized an enzyme, encoded by the lye gene, that catalyzes bacterioruberin biosynthesis. BO-mediated inhibition of bacterioruberin synthesis appears to be specific to the H. salinarum lye-encoded enzyme, as expression of a lye homolog from Haloferax volcanii, a related archaeon that synthesizes bacterioruberins but lacks opsins, resulted in bacterioruberin synthesis that was not reduced in the presence of BO. Our results provide evidence for a novel regulatory mechanism in which biosynthesis of a cofactor is promoted by apoprotein-mediated inhibition of an alternate biochemical pathway. Specifically, BO accumulation promotes retinal production by inhibiting bacterioruberin biosynthesis.
由蛋白质和小分子组成的整合膜蛋白复合物作为辅助因子，在所有生物体中都具有重要功能。为了形成功能性复合物，辅助因子的生物合成必须与相应的脱辅基蛋白的产生相协调。为了研究这种协调，我们研究了嗜盐古菌盐杆菌中的光诱导质子泵菌紫膜（bacteriorhodopsin，BR）。这个复合物由视黄醛辅因子和菌紫质（bacterioopsin，BO），即 BR 的脱辅基蛋白组成。为了研究可能存在的将 BO 和视黄醛生物合成联系起来的新调控机制，我们删除了编码 BO 的 bop 基因。bop 缺失导致菌紫质酮（bacterioruberins）的大量积累，菌紫质酮是与视黄醛共享生物合成前体的类胡萝卜素分子。进一步的研究表明，无论是否存在 BO、视黄醛或 BR，菌紫质酮都会在没有 BO 的情况下积累，这表明 BO 抑制菌紫质酮生物合成，以增加类胡萝卜素前体对视黄醛生物合成的可用性。为了进一步研究这种潜在的调控机制，我们对编码 lye 基因的酶进行了特征描述，该酶催化菌紫质酮生物合成。BO 介导的菌紫质酮合成抑制似乎是针对盐杆菌的 lye 编码酶特异的，因为来自相关古菌盐生盐球菌的 lye 同源物的表达，该古菌合成菌紫质酮但缺乏视蛋白，导致在存在 BO 的情况下菌紫质酮的合成没有减少。我们的研究结果为一种新的调控机制提供了证据，即在这种调控机制中，辅助因子的生物合成是通过脱辅基蛋白介导的对替代生化途径的抑制来促进的。具体而言，BO 的积累通过抑制菌紫质酮生物合成来促进视黄醛的产生。
