PRSS55

PRSS55 is a testis-specific, GPI-anchored chymotrypsin-like serine protease essential for male fertility that functions in sperm maturation, migration through the uterotubal junction, and zona pellucida binding. It contains a catalytic triad (His108, Asp156, Ser250) and localizes to the sperm acrosome and mitochondria; during epididymal maturation it stabilizes ADAM3 protein on sperm, though without direct physical interaction, and its loss abolishes ADAM3 from mature spermatozoa and impairs sperm-egg recognition (PMID:30032357, PMID:32301961). PRSS55 also localizes to mitochondria where it physically interacts with BCKDK and BCKDHA to regulate branched-chain amino acid catabolism and mitochondrial energy production; its absence causes BCAA accumulation, decreased ATP content, reduced mitochondrial membrane potential, and structural defects in mitochondrial cristae and axonemal microtubules (PMID:41444608, PMID:33417308). A homozygous missense mutation (p.A192V) in human PRSS55 causes male infertility with teratozoospermia (PMID:35821214).

1. 2008 Medium

   Initial molecular characterization established that PRSS55 is a membrane-anchored S1-family serine protease with a complete catalytic triad and GPI-anchoring capability, resolving its domain architecture and testicular cell-type expression.

   Evidence RACE cloning, domain analysis, and immunohistochemistry on human testis

   PMID:18844450

   Open questions at the time

   • No enzymatic activity or substrate demonstrated
   • Functional role in reproduction not yet tested
   • GPI-anchoring not confirmed biochemically (inferred from hydrophobic domain)
2. 2018 High

   Knockout studies revealed that PRSS55 is required for sperm migration from uterus to oviduct and for sperm-zona pellucida binding, and that it controls ADAM3 maturation on epididymal sperm without direct physical interaction with ADAM3.

   Evidence Prss55-/- knockout mice with in vivo/in vitro fertilization assays, co-immunoprecipitation, immunofluorescence

   PMID:30032357

   Open questions at the time

   • Mechanism by which PRSS55 stabilizes ADAM3 indirectly remains unknown
   • No proteolytic substrate identified
   • Whether catalytic activity is required for function not tested
3. 2020 High

   Genetic epistasis analysis demonstrated that PRSS55, but not its paralog PRSS51, is the functionally relevant protease for UTJ migration and ZP binding, independently confirming ADAM3 loss as a key downstream consequence.

   Evidence CRISPR/Cas9 single and double knockout mice with fertility trials and sperm migration assays

   PMID:32301961

   Open questions at the time

   • Whether PRSS55 acts enzymatically or as a scaffold/chaperone is unresolved
   • Downstream signaling between PRSS55 loss and ADAM3 destabilization not mapped
4. 2021 Medium

   Complete deletion of Prss55 transcripts revealed sperm ultrastructural defects (disrupted 9+2 axoneme, damaged dense fibers, defective mitochondrial cristae) and decreased ATP, expanding the functional role beyond ADAM3 to mitochondrial and cytoskeletal integrity.

   Evidence Knockout mice with transmission electron microscopy, ATP measurement, and proteomics

   PMID:33417308

   Open questions at the time

   • Proposed myosin activation mechanism is preliminary and not validated by direct interaction or activity assays
   • Relationship between mitochondrial defects and ADAM3 loss not clarified
   • Whether structural defects are primary or secondary to metabolic dysfunction is unclear
5. 2022 Medium

   Identification of a human PRSS55 missense variant (p.A192V) causing teratozoospermia established PRSS55 as a human male infertility gene and demonstrated that protein stability is critical for function.

   Evidence Whole exome sequencing of infertile patient, protein-level validation by western blot and immunofluorescence

   PMID:35821214

   Open questions at the time

   • Single family reported; replication in independent cohorts needed
   • Whether p.A192V disrupts catalytic activity versus protein folding not distinguished
   • No functional rescue experiment performed
6. 2025 Medium

   Transgenic rescue with human PRSS55 confirmed functional conservation across species and showed that C-terminal membrane orientation (intracellular vs. extracellular tag) critically affects fertility restoration.

   Evidence Transgenic rescue in Prss55-/- mice with continuous mating fertility trials

   PMID:40764777

   Open questions at the time

   • Whether partial rescue by GPI-tagged version reflects impaired trafficking, folding, or activity is unknown
   • Single lab study without independent replication
7. 2025 Medium

   Mitochondrial localization and physical interaction with BCKDK and BCKDHA linked PRSS55 to branched-chain amino acid catabolism and mitochondrial energy production, providing a mechanistic basis for the ATP deficiency observed in knockout sperm.

   Evidence Co-IP, LC-MS/MS, metabolomics, ATP and NAD+ assays in knockout mice and HEK293 overexpression

   PMID:41444608

   Open questions at the time

   • Whether PRSS55 cleaves BCKDK/BCKDHA or acts as a non-catalytic scaffold is unresolved
   • Single lab study; BCKDK/BCKDHA interaction not independently replicated
   • How PRSS55 is dually localized to acrosome and mitochondria is unexplained

• Whether PRSS55 functions as an active serine protease or through non-catalytic mechanisms remains unknown; no proteolytic substrate has been identified, catalytic-dead mutants have not been tested, and the structural basis for ADAM3 stabilization and BCAA metabolic regulation is uncharacterized.
• No substrate identified for PRSS55 protease activity
• Catalytic-dead mutant not tested in vivo
• No structural model explaining PRSS55 interaction with BCKDK/BCKDHA or mechanism of ADAM3 stabilization