||Cyclin-dependent protein kinase 5, Cdk5, achieves its full activity only upon the association of its neuronal specific activator, Nck5a or Nck5ai. To date, a variety of cellular proteins have been shown to undergo high affinity association with Nck5a. Three novel Nck5a-associated proteins, C42, C48 and C53 were isolated recently by using yeast two-hybrid system screening. In this MPhil thesis research, a 26-amino acid region of Nck5a was found as a common region which is capable of interacting with these three novel Nck5a-associated proteins and the binding of these Nck5a-associated proteins to Nck5a has no effect on Cdk5 kinase activity. In addition, the 26-amino acid binding region of Nck5a can also interact with Cdk5 with high binding affinity, thus forming ternary complex with Cdk5 and a Nck5a-associated protein. To understand the molecular basis of such interaction, different reaction conditions and site-directed mutagenesis were used during the investigation. Unlike the binding between Cdk5 and Nck5a which is via hydrophobic-hydrophobic interaction, the binging between Nck5a and these Nck5a associated proteins was proved to be charge-charge interaction. Glu157 of Nck5a was proved to be essential for such interactions; mutation of Glu157 to Gln totally abolished the binding of Nck5a and these Nck5a associated proteins.