In
Arabidopsis
, monogalactosyldiacylglycerol (MGDG) is synthesized by a multigenic family of MGDG synthases consisting of two types of enzymes differing in their N-terminal portion: type A (atMGD1) and type B (atMGD2 and atMGD3). The present paper compares type B isoforms with the enzymes of type A that are known to sit in the inner membrane of plastid envelope. The occurrence of types A and B in 16:3 and 18:3 plants shows that both types are not specialized isoforms for the prokaryotic and eukaryotic glycerolipid biosynthetic pathways. Type A
atMGD1
gene is abundantly expressed in green tissues and along plant development and encodes the most active enzyme. Its mature polypeptide is immunodetected in the envelope of chloroplasts from
Arabidopsis
leaves after cleavage of its transit peptide. atMGD1 is therefore likely devoted to the massive production of MGDG required to expand the inner envelope membrane and build up the thylakoids network. Transient expression of green fluorescent protein fusions in
Arabidopsis
leaves and
in vitro
import experiments show that type B precursors are targeted to plastids, owing to a different mechanism. Noncanonical addressing peptides, whose processing could not be assessed, are involved in the targeting of type B precursors, possibly to the outer envelope membrane where they might contribute to membrane expansion. Expression of type B enzymes was higher in nongreen tissues, i.e., in inflorescence (
atMGD2
) and roots (
atMGD3
), where they conceivably influence the eukaryotic structure prominence in MGDG. In addition, their expression of type B enzymes is enhanced under phosphate deprivation.
在拟南芥中,单半乳糖二酰基甘油(MGDG)由由多基因家族的MGDG合成酶合成,由两种不同的酶组成,其N-末端不同:A型(atMGD1)和B型(atMGD2和atMGD3)。本文比较了B型同工酶与已知坐落在质体包膜内膜的A型酶。16:3和18:3植物中A和B型的出现表明,两种类型并不是原核和真核甘油脂生物合成途径的专门同工酶。atMGD1基因在绿色组织和植物发育过程中丰富表达,并编码最活跃的酶。其成熟多肽在转运肽剪切后在拟南芥叶绿体的包膜中被免疫检测到。因此,atMGD1可能专门用于大量生产所需的MGDG,以扩展内包膜膜并建立类囊体网络。在拟南芥叶片中绿色荧光蛋白融合的瞬时表达和体外进口实验表明,B型前体由于不同的机制被定位到质体中。非规范寻址肽参与了B型前体的定位,其加工无法评估,可能定位于外包膜膜,从而有助于膜的扩张。B型酶的表达在非绿色组织,即花序(atMGD2)和根(atMGD3)中更高,它们可能影响MGDG中真核结构的突出性。此外,在缺磷条件下,B型酶的表达得到增强。