Clark A M, Jacobsen K R, Bostwick D E, Dannenhoffer J M, Skaggs M I, Thompson G A
Department of Plant Sciences, University of Arizona, Tucson 86712, USA.
Plant J. 1997 Jul;12(1):49-61. doi: 10.1046/j.1365-313x.1997.12010049.x.
Sieve elements in the phloem of most angiosperms contain proteinaceous filaments and aggregates called P-protein. In the genus Cucurbita, these filaments are composed of two major proteins: PP1, the phloem filament protein, and PP2, the phloem lactin. The gene encoding the phloem filament protein in pumpkin (Cucurbita maxima Duch.) has been isolated and characterized. Nucleotide sequence analysis of the reconstructed gene gPP1 revealed a continuous 2430 bp protein coding sequence, with no introns, encoding an 809 amino acid polypeptide. The deduced polypeptide had characteristics of PP1 and contained a 15 amino acid sequence determined by N-terminal peptide sequence analysis of PP1. The sequence of PP1 was highly repetitive with four 200 amino acid sequence domains containing structural motifs in common with cysteine proteinase inhibitors. Expression of the PP1 gene was detected in roots, hypocotyls, cotyledons, stems, and leaves of pumpkin plants. PP1 and its mRNA accumulated in pumpkin hypocotyls during the period of rapid hypocotyl elongation after which mRNA levels declined, while protein levels remained elevated. PP1 was immunolocalized in slime plugs and P-protein bodies in sieve elements of the phloem. Occasionally, PP1 was detected in companion cells. PP1 mRNA was localized by in situ hybridization in companion cells at early stages of vascular differentiation. The developmental accumulation and localization of PP1 and its mRNA paralleled the phloem lactin, further suggesting an interaction between these phloem-specific proteins.
大多数被子植物韧皮部的筛管分子含有称为P蛋白的蛋白质细丝和聚集体。在南瓜属中，这些细丝由两种主要蛋白质组成：PP1，韧皮部细丝蛋白；PP2，韧皮部乳蛋白。南瓜（Cucurbita maxima Duch.）中编码韧皮部细丝蛋白的基因已被分离和鉴定。对重建基因gPP1的核苷酸序列分析揭示了一个连续的2430 bp蛋白质编码序列，无内含子，编码一个809个氨基酸的多肽。推导的多肽具有PP1的特征，并包含通过对PP1进行N端肽序列分析确定的15个氨基酸序列。PP1的序列高度重复，有四个200个氨基酸的序列结构域，含有与半胱氨酸蛋白酶抑制剂共有的结构基序。在南瓜植株的根、下胚轴、子叶、茎和叶中检测到PP1基因的表达。在南瓜下胚轴快速伸长期间，PP1及其mRNA积累，之后mRNA水平下降，而蛋白质水平保持升高。PP1免疫定位在韧皮部筛管分子的黏液塞和P蛋白体中。偶尔，在伴胞中检测到PP1。在血管分化早期，通过原位杂交将PP1 mRNA定位在伴胞中。PP1及其mRNA的发育积累和定位与韧皮部乳蛋白平行，进一步表明这些韧皮部特异性蛋白之间存在相互作用。
