Model of indiscriminate copolymerization and selective depolymerization of NMIIA and NMIIB explaining self-organization of NMII paralogs into a polarized contractile system in motile cells. Polymerization of NMIIA (red) and NMIIB (blue) monomers occurs with similar efficiency (forward arrows), whereas their dissociation rates (reverse arrows) are different. Faster dissociation of NMIIA subunits together with equivalent addition of new NMIIA and NMIIB subunits leads to gradual accumulation of NMIIB in old filaments during their centripetal drift because of retrograde actin flow.