Tau plays an important role in modulating axonal microtubules in neurons, while intracellular tau aggregates are found in many neurodegenerative disorders. Tubulin binding sites are found in tau's proline-rich region (PRR), microtubule binding repeats (MTBRs), and pseudo-repeat (R'). Tau phosphorylation sites, which cluster with high frequency within the PRR, regulate tubulin interactions and correlates with disease. Here, we use fluorescence correlation spectroscopy and structural mass spectrometry techniques to characterize the impact of phosphomimic mutations in the PRR on tau function. We find that phosphomimics cumulatively diminish tubulin dimer binding and slow microtubule polymerization. Additionally, we map two ∼15 residue regions of the PRR as primary tubulin dimer binding sites and propose a model in which PRR enhances lateral interactions between tubulin dimers, complementing the longitudinal interactions observed for MTBR. Our study provides insight into the previously overlooked relevance of tau's PRR in functional interactions with tubulin dimers.