Enterococcus faecium has six (numbered 1-6), species-specific penicillin binding proteins (PBP) that are either essential or non-essential for growth under certain conditions.  These PBPs are also characterized by varying affinities for beta-lactam agents.  PBP 2 has high affinity for beta-lactam antibiotics.  PBP 4 has low affinity for beta-lactam antibiotics and PBP 5 has very low affinity for beta-lactams. Which of these three PBPs is responsible for beta-lactam resistant in E. faecium?