The intracellular localization and the physiological properties of type 2 phosphatidic acid phosphatase.

脂质磷酸磷酸酶（Lipid Phosphate Phosphatases，LPPs）是一种具有6个跨膜结构域的膜蛋白，可作为胞外酶使多种细胞外脂质磷酸盐脱磷酸化。使用极化MDCK细胞稳定表达人LPP 1和LPP 3，我们发现IPP 1只位于顶面，而LPP 3主要分布在基底外侧亚区。我们在LPP 1的N末端鉴定了一种新的顶端分选信号，由F（2）DKTRL（7）组成。在LPP 3的情况下，存在于第二细胞质部分中的双酪氨酸基序被鉴定为基底外侧靶向信号。我们的工作表明，LPP 1和LPP 3配备了不同的分选信号，使它们差异定位于顶端与基底外侧子域，分别。LPP 1和LPP 3的不同靶向可能是由于它们的不同筏关联。Triton X-100对LPP 1不增溶，但对LPP 3不增溶，而CHAPS从不增溶两种LPP同工酶。蔗糖密度梯度离心法显示LPP 1和LPP 3以不同的方式与脂筏结合。为了了解LPP 1和LPP 3的生理作用，我们研究了LPP表达在人卵巢癌细胞系中的作用。在细胞系中，细胞外溶血磷脂酸（LPA）组成性产生诱导肝素结合EGF样生长因子（HB-EGF）胞外域脱落，EGFR反式激活和癌细胞增殖。LPP 1和LPP 3均能抑制HB-EGF的组成性脱落，提示LPP使细胞外LPA水平保持在较低水平。LPP 1和LPP 3之间不同的细胞内定位是否影响它们的生理作用是未来需要了解的重要问题。

Lipid phosphate phosphatases(LPPs) are integral membrane proteins with six transmembrane domains that act as ectoenzymes dephosphorylating a variety of extracellular lipid phosphates. Using polarized MDCK cells stably expressig human LPP1 and LPP3, we found that IPP1 was located exclusively at the apical surface whereas LPP3 was distributed mostly in the basolateral subdomain. We identified a novel apical sorting signal at the N-terminus of LPP1 composed of F(2) DKTRL(7). In the case of LPP3, a dityrosine motif present in the second cytoplasmic portion was identified as basolateral targeting signal. Our work shows that LPP1 and LPP3 are equipped with distinct sorting signals that cause them to differentially localize to the apical vs. the basolateral subdomain, respectively. The different targeting of LPP1 from LPP3 may be due to their different raft associations. LPP1 not but LPP3 was solubilized with Triton X-100, while CHAPS never solubilized both LPP isozymes. Sucrose density gradient centrifugation assay revealed that LPP1 and LPP3 were associated with lipid rafts in different manners. To understand the physiological roles of LPP1 and LPP3, we investigated the effects of LPP expression in the human ovarian cancer cell lines. In the cell lines, extracellular lysophosphatidic acid(LPA) constitutively produced induce heparin-binding EGF-like growth factor(HB-EGF) ectodomain shedding, EGFR transactivation, and the cancer cell proliferation. Both LPP1 and LPP3 decreased the constitu1tive shedding of HB-EGF, suggesting LPPs kept the extracellular LPA level low. Whether the distinct intracellular localization between LPP1 and LPP3 affects their physiological roles is an important issue to understand in the future.