Ed on the mutations that significantly increased the transduction efficiency over that from our most efficient previously published triple-tyrosine mutant vector. However, it is possible that additional superior combinations could be identified with even more mutations, and our explanation that phosphorylation of specific amino acids in AAV capsid alone accounts for the observed differences may be inadequate. The 17 AAV2 surface-exposed threonine residues are scattered throughout the capsid. Four of the mutations, T329V, T330V, T713V, and T716V, resulted in significant defects in assembly and vector production and could not be further characterized. Residues 329 and 330 are located in the a surface loop (DE loop)located between the bD and bE strands of the core b-barrel of 
the AAV2 VP3 structure [20]. Five of these loops, from icosahedral five-fold symmetry related VP3s assembly a channel at this axis which connects the interior and exterior surfaces of the capsid (Fig. 7a). As was observed in a previous study by Bleker et al. [43], titers for these mutants were significantly reduced consistent with a role for the channel in inhibitor genome packaging. Residues 713 and 716 are located on the wall/raised capsid region between the depressions at and surrounding the icosahedral two- and five-fold axes, respectively (Fig. 7a and b). Their side-chains participate in polar interactions with symmetry related VP3 monomers and it is likely that mutation results in a defect in capsid assembly. A role in capsid assembly for residues located at the icosahedral two-fold axis is consistent with a recent report by Naumer et al. in which they observe that the AAV2 residues which mediate the interaction with the assembly-activating protein (AAP) are located at this capsid region [44]. Residues T455, T491, T550, and T659, showing an increased transduction phenotype when mutated to valine or alanine, are located on the protrusions which surround the icosahedral threefold axis (T455, T491, and T550) or on the HI loop (between bH and bI of the core b-barrel) (T659) which is lies on the depression surrounding the channel at the icosahedral five-fold axis of theLimits of Optimization of Recombinant AAV2 VectorsFigure 4. Kinetics of EGFP expression in H2.35 cell mediated by capsid optimized AAV vectors. (a) EGFP expression analysis at 10457188 16, 24 and 48 h post-infection at MOI of 16103 vgs/cell. (b) Quantification of transduction efficiency of each of the optimized scAAV2 vectors. *P,0.005, **P,0.001 vs. WT AAV2. doi:10.1371/journal.pone.0059142.gAAV2 capsid (Fig. 7). The residues on the protrusion, a prominent Epigenetics feature on the capsid assembled from two VP3 monomers, are located close to the top (455), side facing the two-fold depression (491), and side facing the depression surrounding the five-fold (550), respectively, of the protrusions. This AAV region contains the most variability in sequence and structure, and with the exception of residue 659, the other three threonines residues are located to define VP3 variable regions (VRs) [45]. Along with T659, these residues form a footprint onthe capsid surface that extends over the top 1326631 of the protrusion towards the depression surrounding the icosahedral five-fold axis (Fig. 7a and b). Their surface exposure is consistent with the potential to interact with host molecules, which could include kinases. Interestingly, this footprint is flanked by the residues in the triple-tyrosine mutant, Y444, Y500, and Y730, with T491 located pr.Ed on the mutations that significantly increased the transduction efficiency over that from our most efficient previously published triple-tyrosine mutant vector. However, it is possible that additional superior combinations could be identified with even more mutations, and our explanation that phosphorylation of specific amino acids in AAV capsid alone accounts for the observed differences may be inadequate. The 17 AAV2 surface-exposed threonine residues are scattered throughout the capsid. Four of the mutations, T329V, T330V, T713V, and T716V, resulted in significant defects in assembly and vector production and could not be further characterized. Residues 329 and 330 are located in the a surface loop (DE loop)located between the bD and bE strands of the core b-barrel of the AAV2 VP3 structure [20]. Five of these loops, from icosahedral five-fold symmetry related VP3s assembly a channel at this axis which connects the interior and exterior surfaces of the capsid (Fig. 7a). As was observed in a previous study by Bleker et al. [43], titers for these mutants were significantly reduced consistent with a role for the channel in genome packaging. Residues 713 and 716 are located on the wall/raised capsid region between the depressions at and surrounding the icosahedral two- and five-fold axes, respectively (Fig. 7a and b). Their side-chains participate in polar interactions with symmetry related VP3 monomers and it is likely that mutation results in a defect in capsid assembly. A role in capsid assembly for residues located at the icosahedral two-fold axis is consistent with a recent report by Naumer et al. in which they observe that the AAV2 residues which mediate the interaction with the assembly-activating protein (AAP) are located at this capsid region [44]. Residues T455, T491, T550, and T659, showing an increased transduction phenotype when mutated to valine or alanine, are located on the protrusions which surround the icosahedral threefold axis (T455, T491, and T550) or on the HI loop (between bH and bI of the core b-barrel) (T659) which is lies on the depression surrounding the channel at the icosahedral five-fold axis of theLimits of Optimization of Recombinant AAV2 VectorsFigure 4. Kinetics of EGFP expression in H2.35 cell mediated by capsid optimized AAV vectors. (a) EGFP expression analysis at 10457188 16, 24 and 48 h post-infection at MOI of 16103 vgs/cell. (b) Quantification of transduction efficiency of each of the optimized scAAV2 vectors. *P,0.005, **P,0.001 vs. WT AAV2. doi:10.1371/journal.pone.0059142.gAAV2 capsid (Fig. 7). The residues on the protrusion, a prominent feature on the capsid assembled from two VP3 monomers, are located close to the top (455), side facing the two-fold depression (491), and side facing the depression surrounding the five-fold (550), respectively, of the protrusions. This AAV region contains the most variability in sequence and structure, and with the exception of residue 659, the other three threonines residues are located to define VP3 variable regions (VRs) [45]. Along with T659, these residues form a footprint onthe capsid surface that extends over the top 1326631 
of the protrusion towards the depression surrounding the icosahedral five-fold axis (Fig. 7a and b). Their surface exposure is consistent with the potential to interact with host molecules, which could include kinases. Interestingly, this footprint is flanked by the residues in the triple-tyrosine mutant, Y444, Y500, and Y730, with T491 located pr.