Criscitiello Michael F, Flajnik Martin F
Department of Microbiology and Immunology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
Eur J Immunol. 2007 Oct;37(10):2683-94. doi: 10.1002/eji.200737263.
The discovery of a fourth immunoglobulin (Ig) light (L) chain isotype in sharks has revealed the origins and natural history of all vertebrate L chains. Phylogenetic comparisons have established orthology between this new shark L chain and the unique Xenopus L chain isotype sigma. More importantly, inclusion of this new L chain family in phylogenetic analyses showed that all vertebrate L chains can be categorized into four ancestral clans originating prior to the emergence of cartilaginous fish: one restricted to elasmobranchs (sigma-cart/type I), one found in all cold-blooded vertebrates (sigma/teleost type 2/elasmobranch type IV), one in all groups except bony fish (lambda/elasmobranch type II), and one in all groups except birds (kappa/elasmobranch type III/teleost type 1 and 3). All four of these primordial L chain isotypes (sigma, sigma-cart, lambda and kappa) have maintained separate V region identities since their emergence at least 450 million years ago, suggestive of an ancient physiological distinction of the L chains. We suggest that, based upon unique, discrete sizes of complementarity determining regions 1 and 2 and other features of the V region sequences, the different L chain isotypes arose to provide different functional conformations in the Ig binding site when they pair with heavy chains.
在鲨鱼中发现的第四种免疫球蛋白（Ig）轻（L）链同种型揭示了所有脊椎动物L链的起源和自然史。系统发育比较已确定这种新的鲨鱼L链与独特的非洲爪蟾L链同种型sigma之间存在直系同源关系。更重要的是，在系统发育分析中纳入这个新的L链家族表明，所有脊椎动物的L链可分为软骨鱼出现之前起源的四个祖先家族：一个仅限于板鳃亚纲（sigma-cart/ I型），一个存在于所有冷血脊椎动物中（sigma/硬骨鱼II型/板鳃亚纲IV型），一个存在于除硬骨鱼外的所有类群中（lambda/板鳃亚纲II型），一个存在于除鸟类外的所有类群中（kappa/板鳃亚纲III型/硬骨鱼I型和III型）。这四种原始L链同种型（sigma、sigma-cart、lambda和kappa）自至少4.5亿年前出现以来一直保持着各自独立的V区特征，这表明L链在远古时期就存在生理差异。我们认为，基于互补决定区1和2的独特、离散大小以及V区序列的其他特征，不同的L链同种型在与重链配对时会产生不同的功能构象，从而在Ig结合位点发挥不同作用。
