Catalyzes the ATP-dependent polymerization of arginine and aspartate to multi-L-arginyl-poly-L-aspartic acid (cyanophycin; a water-insoluble reserve polymer). [L-4-(L-arginin-2-N-yl)aspartate](n) + ATP + L-aspartate = [L-4-(L-arginin-2-N-yl)aspartate](n)-L-aspartate + ADP + H(+) + phosphate [L-4-(L-arginin-2-N-yl)aspartate](n)-L-aspartate + ATP + L-arginine = [L-4-(L-arginin-2-N-yl)aspartate](n+1) + ADP + H(+) + phosphate Optimum pH is 8.2. Optimum temperature is 50 degrees Celsius. Homodimer. In the C-terminal section; belongs to the MurCDEF family.