 The mechanism of actin filament initiation by Arp2 / 3 has been disputed . The question is where the complex binds the filament and nucleates a " daughter " filament . Historically two models have been proposed . Recent results , and the balance of opinion in the field , favour the side branching model , in which the Arp2 / 3 complex binds to the side of a pre @-@ existing ( " mother " ) filament at a point different from the nucleation site . Although the field lacks a high @-@ resolution crystal structure , data from electron microscopy , together with biochemical data on the filament nucleation and capping mechanisms of the Arp2 / 3 complex , favour side branching . In the alternative barbed end branching model , Arp2 / 3 only associates at the barbed end of growing filaments , allowing for the elongation of the original filament and the formation of a branched filament . This model , which is based on kinetic analysis and optical microscopy , is decreasingly favoured by the field . 
