Structural basis of asymmetry in the human immunodeficiency virus type 1 reverse transcriptase heterodimer.

The reverse transcriptase from human immunodeficiency virus type 1 is a heterodimer consisting of one 66-kDa and one 51-kDa subunit. The p66 subunit contains both a polymerase and an RNase H domain; proteolytic cleavage of p66 removes the RNase H domain to yield the p51 subunit. Although the polymerase domain of p66 folds into an open, extended structure containing a large active-site cleft, that of p51 is closed and compact. The connection subdomain, which lies between the polymerase and RNase H active sites in p66, plays a central role in the formation of the reverse transcriptase heterodimer. Extensive and very different intra- and intersubunit contacts are made by the connection subdomains of each of the subunits. Together, contacts between the two connection domains constitute approximately one-third of the total contacts between subunits of the heterodimer. Conversion of an open p66 polymerase domain structure to a closed p51-like structure results in a reduction in solvent-accessible surface area by 1600 A2 and the burying of an extensive hydrophobic surface. Thus, the monomeric forms of both p66 and p51 are proposed to have the same closed structure as seen in the p51 subunit of the heterodimer. The free energy required to convert p66 from a closed p51-like structure to the observed open p66 polymerase domain structure is generated by the burying of a large, predominantly hydrophobic surface area upon formation of the heterodimer. It is likely that the only kind of dimer that can form is an asymmetric one like that seen in the heterodimer structure, since one dimer interaction surface exists only in p51 and the other only in p66. We suggest that both p51 and p66 form asymmetric homodimers that are assembled from one subunit that has assumed the open conformation and one that has the closed structure. IMAGES:

人类免疫缺陷病毒1型（HIV-1）的逆转录酶（reverse transcriptase）是由1个66 kDa亚基与1个51 kDa亚基组成的异二聚体（heterodimer）。
p66亚基同时包含聚合酶结构域与核糖核酸酶H（RNase H）结构域；对p66进行蛋白水解切割可去除其RNase H结构域，从而得到p51亚基。
尽管p66的聚合酶结构域可折叠为带有大型活性位点裂隙的开放延伸构象，p51的聚合酶结构域则呈现封闭且紧凑的构象。
位于p66聚合酶与RNase H活性位点之间的连接亚结构域（connection subdomain），在逆转录酶异二聚体的组装过程中发挥核心作用。
两个亚基各自的连接亚结构域分别形成了差异显著的广泛亚内与亚基间相互作用界面。
两个连接结构域之间的相互作用总计约占该异二聚体亚基间总相互作用界面的三分之一。
将开放型p66聚合酶结构域转化为类似p51的封闭结构后，其溶剂可及表面积（solvent-accessible surface area）将减少1600 Ų，并掩埋了一处大范围的疏水表面。
因此，学界推测p66与p51的单体形式均采用与该异二聚体p51亚基一致的封闭构象。
使p66从类似p51的封闭结构转化为已观测到的开放型p66聚合酶结构域所需的自由能，源于异二聚体形成过程中一处以疏水成分为主的大范围表面被掩埋所释放的能量。
可以形成的二聚体大概率仅为异二聚体结构中所见的不对称二聚体，因为其中一个二聚体相互作用界面仅存在于p51亚基中，而另一个则仅存在于p66亚基中。
我们提出，p51与p66均可形成不对称同二聚体（homodimer），该同二聚体由一个处于开放构象的亚基与一个处于封闭构象的亚基组装而成。
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