IZUMO4 notes

Deep research (falcon / perplexity-lite) failed in this environment; notes compiled from cached
publications, PubMed search and UniProt (Q1ZYL8).

• Izumo family member with IZUMO domain; signal peptide 1-15, no TM segment; UniProt "Secreted" (ECO:0000305).
• Soluble, broader tissue expression than IZUMO1-3 PMID:19658160.
• Lost after acrosome reaction in mouse sperm, suggesting intra-acrosomal PMID:19658160.
• Forms a dimer-sized complex on sperm (PMID:19658160).
• Human sperm: immunolocalized to acrosomal region; anti-IZUMO4 antibodies reduce cumulus penetration, hemizona binding and sperm penetration assays PMID:40760720.
• Mouse Izumo4 KO males fertile PMID:40572022.
• Nucleus HDA (PMID:21630459, sperm nucleus proteomics) is weak; ARBA membrane IEA wrong (no TM).

Localization conclusion: soluble secretory-pathway protein, acrosomal content (released at acrosome reaction);
not an integral plasma-membrane protein.