Eirín-López José María, Frehlick Lindsay J, Chiva Manel, Saperas Núria, Ausió Juan
Department of Biochemistry and Microbiology, University of Victoria, Victoria, British Columbia, Canada.
Mol Biol Evol. 2008 Aug;25(8):1705-13. doi: 10.1093/molbev/msn121. Epub 2008 May 23.
The sperm nuclear basic proteins (SNBPs) that participate in chromatin condensation in spermatozoa belong to 3 groups: histone (H), protamine-like (PL), and protamine (P) type. They share a common origin with histone H1 resulting from the segregation of PL components, corresponding to different regions of an H1 precursor molecule (N-terminal, winged-helix, C-terminal domains), becoming independent and following a subsequent process of parallel vertical evolution (H <--> PL <--> P). In the present work, we describe the sequence and primary structure of the main SNBP component in the sperm of the cephalochordate Branchiostoma floridae (amphioxus), revealing that it represents the deuterostome counterpart of the PL-III SNBP component from molluscs corresponding to the H1 N-terminal region. Until now, this has been a missing piece needed to complete the evolutionary history of SNBPs in metazoan genomes. The discovery of this PL lineage in deuterostomes definitively validates the parallel vertical evolution of SNBPs across metazoans, giving further support to the "basal" position of amphioxus among chordates, with respect to tunicates. Sequence analyses suggest that later on in evolution, the appearance of positively selected arginine-rich protamines, derived from the H1 C-terminal region, led to the extinction of this PL lineage in the genomes of early protostomes and deuterostomes. Given that tunicates are now viewed as a sister group of vertebrates, the lysine to arginine transition responsible for the origin of vertebrate protamines must be set a step back from tunicates.
参与精子染色质凝聚的精子核碱性蛋白（SNBPs）可分为3类：组蛋白（H）、类鱼精蛋白（PL）和鱼精蛋白（P）型。它们与组蛋白H1有着共同的起源，是由PL成分分离而来，对应于H1前体分子的不同区域（N端、翼状螺旋、C端结构域），之后各自独立，并经历了一个平行垂直进化的过程（H <--> PL <--> P）。在本研究中，我们描述了头索动物佛罗里达文昌鱼（文昌鱼）精子中主要SNBP成分的序列和一级结构，发现它代表了软体动物中对应于H1 N端区域的PL-III SNBP成分的后口动物对应物。到目前为止，这一直是完成后生动物基因组中SNBPs进化史所需的缺失环节。在头索动物中发现这个PL谱系最终证实了SNBPs在整个后生动物中的平行垂直进化，进一步支持了文昌鱼相对于被囊动物在脊索动物中的“基础”地位。序列分析表明，在进化后期，源自H1 C端区域的富含精氨酸的正选择鱼精蛋白的出现，导致了这个PL谱系在早期原口动物和后口动物基因组中的灭绝。鉴于现在将被囊动物视为脊椎动物的姐妹群，脊椎动物鱼精蛋白起源所涉及的赖氨酸到精氨酸的转变必须比被囊动物再往前推一步。
