PTDSS2

PTDSS2 (PSS2) is a phosphatidylserine synthase that produces phosphatidylserine (PS) through a serine base-exchange reaction, contributing to membrane lipid composition with downstream consequences for fertility, immune evasion, and viral budding (PMID:10432300, PMID:12361952). The purified enzyme converts phosphatidylethanolamine (PE), but not phosphatidylcholine, into PS, and is directly inhibited by its PS product—a feedback regulation that distinguishes it from PSS1 (PMID:10432300, PMID:19014349). PTDSS2 displays marked substrate selectivity, preferring DHA (22:6n-3) at the sn-2 position of PE and thereby preferentially generating DHA-containing PS, and it favors diacyl PE over PE plasmalogen (PMID:23071296). The enzyme localizes to mitochondria-associated membranes and is largely excluded from bulk endoplasmic reticulum (PMID:10938271). In vivo, PTDSS2 accounts for the great majority of serine-exchange activity in testis, brain, and liver, yet its loss leaves bulk phospholipid content unchanged and does not trigger compensatory PSS1 up-regulation; knockout mice show reduced testis weight and male infertility, consistent with high expression in Sertoli cells (PMID:12361952). PTDSS2-dependent PS synthesis is dispensable for PS externalization during apoptosis (PMID:14984733) but supports nonapoptotic PS exposure at the plasma membrane that promotes immunosuppression and breast cancer metastasis and is required for efficient Ebola virus budding (PMID:30289506, PMID:35025764). A gain-of-function R97K mutant, analogous to Lenz-Majewski syndrome PSS1 mutations, impairs osteoclast formation and remodels cellular phospholipid composition (PMID:37714410).

1. 1999 High

   Established the core catalytic identity of PTDSS2 by showing it uses PE for serine base-exchange and, unlike PSS1, is subject to product feedback inhibition, defining a distinct regulatory mode for this synthase.

   Evidence Stable cDNA expression in hepatoma and CHO cells with in vitro serine-exchange assays, metabolic labeling, and ethanolamine auxotrophy rescue

   PMID:10432300

   Open questions at the time

   • Substrate preference among PE species not yet resolved
   • Structural basis of product inhibition unknown
2. 2000 High

   Resolved where PTDSS2 acts within the cell, placing it at mitochondria-associated membranes rather than bulk ER, situating PS synthesis at an organelle contact site.

   Evidence Subcellular fractionation and immunoblotting of endogenous and recombinant PSS2 in CHO-K1 and McArdle cells

   PMID:10938271

   Open questions at the time

   • Mechanism of MAM targeting not defined
   • Functional consequence of MAM localization for PS trafficking unaddressed
3. 2002 High

   Demonstrated the in vivo contribution of PTDSS2 to tissue serine-exchange activity and its physiological role, showing it accounts for most activity in testis, brain, and liver and that its loss impairs male fertility without altering bulk phospholipid content.

   Evidence Pss2 knockout mouse with tissue serine-exchange assays, phospholipid analysis, beta-galactosidase reporter localization, and fertility assessment

   PMID:12361952

   Open questions at the time

   • Why phospholipid content is buffered despite >90% activity loss is unexplained
   • Mechanistic link between PS synthesis and Sertoli cell/fertility function not established
4. 2004 High

   Tested whether PS synthases drive apoptotic PS externalization and showed they do not, dissociating the bulk PS biosynthetic machinery from the canonical apoptotic 'eat-me' signal.

   Evidence Staurosporine-induced apoptosis in PSS1- and/or PSS2-deficient CHO cells with annexin V flow cytometry and DNA fragmentation

   PMID:14984733

   Open questions at the time

   • Source of PS externalized during apoptosis not identified
   • Does not address nonapoptotic PS exposure
5. 2009 High

   Confirmed substrate specificity and product inhibition directly with purified human enzyme, removing ambiguity from cell-based assays.

   Evidence Purified epitope-tagged human PSS2 in in vitro assays with defined substrates and exogenous PS

   PMID:19014349

   Open questions at the time

   • Kinetic parameters and inhibition constants not detailed here
   • No structural model of substrate/product binding
6. 2012 High

   Defined acyl-chain selectivity, showing PTDSS2 preferentially uses DHA-containing PE and diacyl over plasmalogen PE, explaining how it shapes the DHA-rich PS pool.

   Evidence Immunopurified Flag-tagged PSS2 from HEK cells assayed with defined substrate species, plus mutant cell microsomes

   PMID:23071296

   Open questions at the time

   • Structural determinant of DHA preference unknown
   • In vivo relevance of acyl preference to specific tissues not tested
7. 2018 Medium

   Linked PTDSS2-generated PS to viral egress by showing combined PTDSS1/PTDSS2 knockdown blocks Ebola virus budding at the plasma membrane.

   Evidence siRNA co-knockdown of PTDSS1 and PTDSS2 with viral production assay and electron microscopy

   PMID:30289506

   Open questions at the time

   • PTDSS1 and PTDSS2 knocked down together, so individual PTDSS2 contribution not isolated
   • Single lab, single virus
8. 2022 Medium

   Connected PTDSS2 expression to nonapoptotic surface PS exposure that drives tumor immune evasion and metastasis, and placed it downstream of BRCA1 regulation.

   Evidence Expression manipulation, PS externalization assays, in vivo mouse metastasis model, and tumor microenvironment analysis

   PMID:35025764

   Open questions at the time

   • Mechanism by which BRCA1 represses PTDSS2 not defined
   • Single lab; direct causality of surface PS in immunosuppression partly correlative
9. 2023 Medium

   Showed a gain-of-function PSS2 mutation analogous to Lenz-Majewski PSS1 mutations remodels cellular phospholipid composition and impairs osteoclast formation, implicating PTDSS2 activity in bone cell biology.

   Evidence Expression of PSS2 R97K gain-of-function mutant in osteoclast precursors with differentiation assays, podosome imaging, and lipidomics; catalytically inactive control

   PMID:37714410

   Open questions at the time

   • No human disease attributable to PTDSS2 mutation demonstrated
   • Mechanism linking lipid changes to osteoclast defect unresolved
   • Single lab

• How PTDSS2-derived PS is selectively trafficked from MAM to the plasma membrane outer leaflet, and what controls its tissue-specific physiological versus disease-associated roles, remains unresolved.
• No defined PS transport pathway from MAM to plasma membrane in the corpus
• No structural model of the enzyme
• Regulators beyond product inhibition and BRCA1 largely uncharacterized