ACTR1B: does beta-centractin retain the Arp1 nucleotide site and

protomer interface?

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What was measured

Contact residues were derived from PDB 9B85 (cryo-EM structure of human dynactin bound to the Chlamydia effector Dre1, released 2025-04-16), not recited for conventional actin by analogy.
All 8 Arp1 filament protomers in that entry (chains A, B, C, D, E, F, G, I) are modelled as ACTR1A / alpha-centractin, each with a bound ADP. ACTR1B is assigned to 0 chains of 9B85 by SIFTS, and PDBe maps 0 PDB entries to P42025 in total — i.e. ACTR1B is present in no deposited structure at all, so its residues are tested by alignment onto the ACTR1A protomer.

Cutoff: heavy-atom contacts within 4.0 A. Reference chain A maps 1:1 onto P61163 residues 1-376 by SIFTS (re-verified at runtime).

nucleotide_site (18 residues): residues of Arp1 chain A within 4.0 A of its bound ADP
protomer_interface (37 residues): residues of Arp1 chain A within 4.0 A of a neighbouring Arp1 protomer (chains C,B)

Retention per sequence

Accession	Role	Len	% id to ACTR1A	Nucleotide site	Protomer interface
P42025	ACTR1B human (beta-centractin) -- the gene under review	376	90.4	17/18	32/37
P61163	ACTR1A human (alpha-centractin) -- paralog, modelled in 9B85	376	100.0	18/18	37/37
F2Z5G5	ACTR1A pig -- WITH/FROM donor on the IBA rows	349	90.3	17/18	37/37
O94630	arp1 S. pombe -- WITH/FROM donor (sole donor for GO:0106006)	379	52.0	13/18	20/37
P38696	ARP1 S. cerevisiae -- WITH/FROM donor	384	53.2	10/18	20/37
Q9NA98	arp-1 C. elegans -- WITH/FROM donor (TrEMBL)	374	70.3	14/18	27/37
Q5BBX7	AN1953 A. nidulans -- WITH/FROM donor (TrEMBL)	380	63.8	12/18	25/37
Q8R5C5	Actr1b mouse -- ortholog of the gene under review	376	91.2	17/18	32/37
P60709	ACTB human (beta-actin) -- conventional actin outgroup	375	53.6	11/18	16/37
Q9NZ32	ACTR10 human (Arp11) -- dynactin pointed-end capping Arp	417	29.5	6/18	10/37

Substitutions, per sequence

P42025 — ACTR1B human (beta-centractin) -- the gene under review

nucleotide_site: 17/18 identical; substitutions: K215->R215
protomer_interface: 32/37 identical; substitutions: V45->M45, Y197->L197, S207->T207, I213->V213, I268->V268

P61163 — ACTR1A human (alpha-centractin) -- paralog, modelled in 9B85

nucleotide_site: 18/18 identical; substitutions: none
protomer_interface: 37/37 identical; substitutions: none

F2Z5G5 — ACTR1A pig -- WITH/FROM donor on the IBA rows

nucleotide_site: 17/18 identical; substitutions: L337->gap
protomer_interface: 37/37 identical; substitutions: none

O94630 — arp1 S. pombe -- WITH/FROM donor (sole donor for GO:0106006)

nucleotide_site: 13/18 identical; substitutions: V20->F20, S191->T191, K215->R215, F307->L311, L337->M341
protomer_interface: 20/37 identical; substitutions: P42->I42, V45->E45, A49->P49, G50->S50, A51->S51, L52->I52, H65->L65, Y197->Q197, E201->S201, F205->L205, H206->V206, G246->N250, S247->Q251, P253->T257, D266->E270, L267->M271, I268->M272

P38696 — ARP1 S. cerevisiae -- WITH/FROM donor

nucleotide_site: 10/18 identical; substitutions: V20->I23, D162->E165, S191->T194, K215->R219, S304->T313, L306->T315, F307->L316, L337->K346
protomer_interface: 20/37 identical; substitutions: P42->T45, H44->Y47, V45->D48, R46->K49, A49->L52, G50->E53, A51->G54, H65->L68, Y197->Q200, E201->S204, F205->L209, H206->F210, K238->G242, S247->R256, P253->N262, D266->Q275, L267->I276

Q9NA98 — arp-1 C. elegans -- WITH/FROM donor (TrEMBL)

nucleotide_site: 14/18 identical; substitutions: S191->T190, K215->R214, S304->T302, L337->N335
protomer_interface: 27/37 identical; substitutions: H44->Y43, V45->H44, A51->G50, H65->N64, Y197->L196, K200->R199, S207->R206, G246->T244, S247->T245, P253->N251

Q5BBX7 — AN1953 A. nidulans -- WITH/FROM donor (TrEMBL)

nucleotide_site: 12/18 identical; substitutions: V20->T19, K22->R21, S191->T189, K215->R213, F307->C312, L337->K342
protomer_interface: 25/37 identical; substitutions: V45->P44, A51->G50, H65->L64, Y197->L195, E201->T199, F205->L203, S207->T205, I213->V211, K238->H243, S247->H252, P253->Q258, D266->E271

Q8R5C5 — Actr1b mouse -- ortholog of the gene under review

nucleotide_site: 17/18 identical; substitutions: K215->R215
protomer_interface: 32/37 identical; substitutions: V45->M45, Y197->L197, S207->T207, I213->V213, I268->V268

P60709 — ACTB human (beta-actin) -- conventional actin outgroup

nucleotide_site: 11/18 identical; substitutions: V20->M16, S191->T186, K215->R210, S304->T303, L306->M305, F307->Y306, L337->K336
protomer_interface: 16/37 identical; substitutions: K43->R39, V45->Q41, R46->G42, A49->V45, A51->M47, L52->G48, H65->K61, L68->I64, Y197->I192, R199->T194, K200->E195, E201->R196, H206->T201, S207->T202, S208->T203, K238->S233, S247->Q246, P253->N252, D266->S265, L267->F266, I268->L267

Q9NZ32 — ACTR10 human (Arp11) -- dynactin pointed-end capping Arp

nucleotide_site: 6/18 identical; substitutions: S18->E22, G19->A23, V20->F24, D162->Y143, G163->R144, S191->H172, K215->E208, E219->A212, S304->T309, L306->M311, F307->L312, L337->A350
protomer_interface: 10/37 identical; substitutions: H44->P51, M48->V55, A49->Q56, G50->Y57, A51->N58, L52->I59, H65->gap, R66->gap, G67->gap, L68->F71, Y197->Q178, R199->L180, K200->E181, E201->Q182, G202->T184, F205->T187, H206->S188, S207->V189, S208->A190, I213->V206, K238->N245, P244->gap, S247->E253, P253->S261, L267->gap, I268->gap, G269->gap

Every contact position where ACTR1B differs from ACTR1A

6 of 55 contact positions differ. 6/6 are conserved in the mouse ACTR1B orthologue (Q8R5C5), so they are fixed features of the beta paralog rather than human-specific noise. At 4/6 the ACTR1B residue is independently present in another Arp1 family member or in conventional actin -- a position the family already tolerates, not a novel loss. The two counts are kept separate because a substitution shared only with the mouse orthologue is not independent evidence of tolerance.

Set	Pos	ACTR1A	ACTR1B	F2Z5G5	O94630	P38696	Q9NA98	Q5BBX7	Q8R5C5	P60709	Q9NZ32	family members (excl. mouse ACTR1B) sharing it
nucleotide_site	215	K	R	K	R	R	R	R	R	R	E	O94630, P38696, P60709, Q5BBX7, Q9NA98
protomer_interface	45	V	M	V	E	D	H	P	M	Q	V	none
protomer_interface	197	Y	L	Y	Q	Q	L	L	L	I	Q	Q5BBX7, Q9NA98
protomer_interface	207	S	T	S	S	S	R	T	T	T	V	P60709, Q5BBX7
protomer_interface	213	I	V	I	I	I	I	V	V	I	V	Q5BBX7, Q9NZ32
protomer_interface	268	I	V	I	M	I	I	I	V	L	-	none

Contact residues (ACTR1A / P61163 numbering)

nucleotide_site: G17, S18, G19, V20, K22, G161, D162, G163, G187, S191, K215, K218, E219, G303, S304, L306, F307, L337
protomer_interface: P42, K43, H44, V45, R46, V47, M48, A49, G50, A51, L52, H65, R66, G67, L68, Y197, R199, K200, E201, G202, F205, H206, S207, S208, E210, I213, K238, L243, P244, D245, G246, S247, P253, D266, L267, I268, G269