ARP-T1 and ARP-T2 were discovered as two major acidic components of the cytoskeletal calyx purified from bull spermatozoa, identified by partial amino acid sequencing as novel actin-related proteins.

Antibodies against the human proteins showed testis-specific synthesis late in spermatid differentiation and localisation in the calyx. The abstract does not state which species was used for the immunofluorescence.

The calyx is defined biochemically by resistance to high ionic strength and detergents, and its stated protein composition before this paper was calicin, cylicin I and II and two major actin-capping proteins.

The authors' own interpretation points away from actin-like behaviour - they note that the calyx is non-motile and suggest these Arps serve functions other than nucleating actin filaments.

The cDNAs for hArpM1 (ACTRT3) and hArpM2 (ACTRT2) were identified and reported to be remarkably similar to conventional actin, with actin's ATP-binding motif and nuclear-export signals highly conserved. This review confirms the ATP-binding claim from structure-derived contacts.

The mRNAs were reported in all tested human tissues at lower abundance than actin, which is compatible with the testis-restricted protein reported elsewhere.

The paper's closing inference that these proteins are involved in cytoskeletal organisation is an explicit speculation from sequence inspection, made before any localisation was known.

ACTRT1, ACTRT2, ACTL7A and ACTL9 interact to form a multimeric complex localising to the subacrosomal region of spermatids.

The complex localises to the subacrosomal region of spermatids, one of the two regions GO's perinuclear theca definition names.

The authors conclude that this sperm perinuclear-theca-specific ARP complex mediates the connection between the acrosome and the nucleus.

Co-immunoprecipitation showed ACTRT3 interacting with ACTRT1, ACTRT2, ACTL7A, SPEM2 and ZPBP, extending the perinuclear-theca complex to five actin-related proteins.

The authors interpret this as ACTRT3 being part of a three-dimensional perinuclear-theca scaffold.

Calicin interacts with itself and many other perinuclear-theca components and may act as an organising centre for theca assembly. Mouse Actrt2's own GO:0033011 IDA and its only curated protein interaction, with calicin (Q8CDE2), both come from this paper.

Calicin redistributes during spermiogenesis, from around the acrosome to around the whole nucleus and finally to the postacrosomal region - the same kind of stage-dependent redistribution that reconciles ACTRT2's subacrosomal and posterior localisations.

In human spermatozoa, ACTRT2 was localised by immunofluorescence to the post-acrosomal region and the middle piece, and its abundance was reduced in obesity-associated asthenozoospermia.

Actrt2-null mice have significantly shrunken seminiferous tubules, and spermatogenesis in heterozygotes is significantly reduced after busulfan treatment. This is the only loss-of-function phenotype reported for this gene.

Reducing ACTRT2 in a spermatogonial cell line increased busulfan-induced death, with reactive-oxygen accumulation and mitochondrial changes characteristic of ferroptosis.

The proposed mechanism is intracellular iron overload with mitochondrial damage, read out as expression changes in ferroptosis regulators rather than as any activity of ACTRT2.

Actin's ATP hydrolysis is triggered by flipping of the His161 side chain toward ATP, which occurs as part of the conformational change from monomeric G-actin to filamentous F-actin.

The flip rearranges the active-site waters, including the attacking water W1, into an orientation capable of hydrolysis.

Gln137 and His161 are the two active-site residues implicated in actin's hydrolysis, and the rotameric state of His161 is governed by the Pro-rich loop at Ala108/Pro109. ACTRT2 retains Gln137 but has lost His161, Ala108 and Pro109.

Actin's ATPase activity operates at the catalytic site in the filamentous F-form, and the G-to-F transition shifts Gln137 and His161 to relocate the attacking and helping waters.

BioPlex 3.0, the source of ACTRT2's only experimentally coded annotation. In this dataset ACTRT2 co-purifies with PDCL3 and with six CCT/TRiC chaperonin subunits, and PDCL3 in the same dataset partners with ACTRT1, ACTRT3, ACTBL2, ACTA2 and the POTE actins.

The provider's own GO grounding proposes structural molecule activity together with cytoskeletal protein binding as ACTRT2's molecular activity. The first is the parent term that PAINT independently chose for the ACTL7A/ACTL7B clade, so it is a third voice for pitching the structural claim at the general level; the second is a fold-to-activity leap, since no actin-binding or cytoskeletal-protein-binding measurement exists for ACTRT2.

The only subcellular-location statement in the entry is a by-similarity one, with no source entry cited, which is what the GO:0005856 keyword row is derived from.

The entry records ARPM2 as the gene synonym and Actin-related protein M2 as an alternative name, which is how the 2001 cloning paper's hArpM2 was identified as this gene rather than as ACTRT3 (ARPM1).

ACTRT2 is identical to actin at all ten literature-named phosphate-loop, cation-site and sensor positions (D11, S14, G15, K18, Q137, D154, G156, D157, V159, R183), confirming the 2001 sequence-inspection claim where it matters most; the adenine/ribose region has diverged, with two non-conservative substitutions among the 19 computed ATP contacts. Retaining the pocket makes nucleotide binding untested rather than demonstrated.

All five reported members of the sperm perinuclear-theca ARP complex have lost His161, while every panel member that extends or nucleates a filament retains it; ACTL7B, which is testis-specific but not a reported complex member, retains it too.

ACTRT2's protomer-protomer interface is below every filament builder in the panel but not below the Arp2/3 subunits, so the measurement argues against filament extension without excluding an Arp2/3-like role.

Every resolved WITH/FROM source on both IBA rows carries its own experimental evidence for the term it donated, so any objection to those rows must concern propagation rather than donor quality.

ACTRT2 and ACTRT3 are the only reported perinuclear-theca complex members with no experimental biological-process annotation in either human or mouse.

ACTRT2's GO:0015629 IBA row is byte-identical to ACTL8's and its GO:0005200 IBA row is byte-identical to ACTR10's, so those two merged reviews' verdicts are verdicts on the same rows and must agree.

PMID:33961781 is the source of 9514 GOA annotations, all of them GO:0005515 and all assigned by IntAct, so the term the row carries conveys nothing specific to ACTRT2.

The mouse GO:0033011 IDA that both perinuclear-theca rows descend from is not a blanket projection: the curator gave the term to 12 of the 19 entities the calicin paper touches, so it was assigned per protein.