免疫球蛋白的基因结构

<font><font><span> </span> <span>编码轻链和重链的基因分布在不同的染色体上，如人的丸链基因位于第</span> <span>22</span> <span>号染色体，κ链基因位于第</span> <span>2</span> <span>号染色体；小鼠的则分别位于第</span> <span>16</span> <span>和第</span> <span>6</span> <span>号染色体。</span> <span>H</span> <span>链位于人的第</span> <span>14</span> <span>号染色体，小鼠的第</span> <span>12</span> <span>号染色体。每条链由不同的基因簇组成，如重链上有</span> <span>L</span> <span>、</span> <span>y</span> <span>、</span> <span>D</span> <span>、</span> <span>J</span> <span>和</span> <span>C</span> <span>基因簇，每种基因簇含有许多个不同的编码基因。</span> </font> </font>

<font><font><span> 1</span> <span>．轻链基因结构</span> <span> </span> <span>轻链有κ链和λ链两种，它们有不同的基因结构。</span> </font> </font>

<font><font><span> (1)</span> <span>κ链的基因结构</span> <span> </span> <span>胚系</span> <span>DNA</span> <span>的κ链基因由</span> <span>V</span> <span>κ、</span> <span> J</span> <span>κ和</span> <span>C</span> <span>κ三类基因簇组成</span> <span>。小鼠的</span> <span>V</span> <span>κ。基因簇约有</span> <span>350</span> <span>个编码基因。每个</span> <span>V</span> <span>区基因上游都有一个</span> <span>L</span> <span>基因编码</span> <span>22</span> <span>个氨基酸的κ链信号肽，信号肽为疏水序列，有利于肽通过内质网向外运输，在κ链合成后，信号肽便自行脱落。</span> <span>v</span> <span>基因编码κ链的</span> <span>96</span> <span>个氨基酸构成的</span> <span>V</span> <span>区蛋白质。</span> <span>J</span> <span>基因簇在</span> <span>y</span> <span>基因簇下游</span> <span>23kb</span> <span>处，由</span> <span>5</span> <span>个基因组成，其中</span> <span>J</span> <span>κ</span> <span>3</span> <span>是无效基因。</span> <span>j</span> <span>基因之间相距较近，每个</span> <span>J</span> <span>，基因约为</span> <span>30</span> <span>多个核苷酸，编码</span> <span>13</span> <span>个氨基酸的肽段。这些氨基酸也属于</span> <span>V</span> <span>区肽段的一部分，称为连接片段。</span> <span>J</span> <span>区下游</span> <span>2</span> <span>．</span> <span>5 kb</span> <span>处为</span> <span>C</span> <span>区基因，只有一个</span> <span>C</span> <span>基因，编码κ链的</span> <span>C</span> <span>区肽段。</span> </font> </font>

<font><font><span> (2)</span> <span>λ链的基因结构</span> <span> </span> <span>胚胎</span> <span>DNA</span> <span>的入链由</span> <span>V</span> <span>λ、人和</span> <span>C</span> <span>λ三类基因组成</span> <span>(</span> <span>图</span> <span>6</span> <span>—</span> <span>26)</span> <span>，每个基因的上游也有</span> <span>L</span> <span>基因。但是，小鼠的入链基因只有</span> <span>V</span> <span>λ</span> <span>1</span> <span>和</span> <span>V</span> <span>λ</span> <span>2</span> <span>两个</span> <span>V</span> <span>基因，两个</span> <span>L</span> <span>基因即</span> <span>L</span> <span>λ</span> <span>1</span> <span>和</span> <span>L</span> <span>λ</span> <span>2</span> <span>在</span> <span>y</span> <span>基因上游。了基因有</span> <span>4</span> <span>个，</span> <span>C</span> <span>基因也有</span> <span>4</span> <span>个，其中</span> <span>J</span> <span>λ</span> <span>4</span> <span>和</span> <span>C</span> <span>λ</span> <span>4</span> <span>为无效基因，其余基因成簇排列。</span> <span>J</span> <span>λ和</span> <span>C</span> <span>λ有</span> <span>1</span> <span>．</span> <span>2 kb</span> <span>的内含子，</span> <span>L</span> <span>基因编码λ链</span> <span>N</span> <span>端的</span> <span>15</span> <span>～</span> <span>20</span> <span>氨基酸的信号肽。</span> <span>V</span> <span>λ</span> <span>基因编码λ链</span> <span>V</span> <span>区</span> <span>1</span> <span>～</span> <span>97</span> <span>位氨基酸，</span> <span>j</span> <span>基因编码λ链</span> <span>V</span> <span>区</span> <span>98</span> <span>～</span> <span>110</span> <span>位氨基酸，</span> <span>C</span> <span>λ基因编码久链全部</span> <span>C</span> <span>区，即</span> <span>111</span> <span>～</span> <span>214</span> <span>位氨基酸。人的免疫球蛋白入链基因结构与小鼠相似，只是</span> <span>y</span> <span>基因约有</span> <span>100</span> <span>个排列在一起，而</span> <span>6</span> <span>个</span> <span>J</span> <span>λ和</span> <span>6</span> <span>个</span> <span>C</span> <span>λ基因呈簇排列。</span> </font> </font>

<font><font><span> 2</span> <span>．重链基因结构</span> <span> </span> <span>重链基因结构与轻链相似，但更为复杂。编码可变区的基因除</span> <span>L</span> <span>、</span> <span>v</span> <span>和</span> <span>J</span> <span>之外，还有</span> <span>D</span> <span>基因。</span> <span>v</span> _{<span>H</span>} <span>基因编码重链</span> <span>V</span> <span>区</span> <span>1</span> <span>～</span> <span>94</span> <span>位氨基酸，</span> <span>95</span> <span>～</span> <span>97</span> <span>位氨基酸由</span> <span>D</span> <span>基因编码，</span> <span>J</span> _{<span>H</span>} <span>基因则编码</span> <span>98</span> <span>～</span> <span>113</span> <span>位氨基酸。</span> <span>D</span> <span>基因位于</span> <span>v</span> _{<span>H</span>} <span>和</span> <span>J</span> _{<span>H</span>} <span>之间。</span> </font> </font>

(责任编辑：大汉昆仑王)