Protein phosphorylation, a crucial post-translational modification, plays a fundamental role in regulating various cellular processes such as cell signaling, metabolism, and gene expression. The human genome encodes approximately 520 protein kinases, which catalyze the addition of phosphate groups to specific amino acid residues on target proteins. The activity of these kinases and their substrate specificity are tightly regulated, influencing cellular functions including cell proliferation, differentiation, and apoptosis.
One critical aspect of substrate specificity lies in the ability of kinases to recognize "linear sequence motifs" within their substrate proteins. These motifs, short and unstructured, contain conserved residues that facilitate interaction with the kinase. Recent advancements in mass spectrometry-based proteomics have enabled the identification of thousands of in vivo phosphorylation sites, yet many of these sites lack characterization regarding their associated kinases and signaling contexts.
Given the increasing importance of mapping the phosphoproteome and the limitations in experimental approaches, there arises a need for robust and efficient in silico methods to predict kinase-specific phosphorylation sites. Various prediction methods exist, ranging from simple consensus patterns to sophisticated machine-learning algorithms. However, the selection of an appropriate prediction method requires careful consideration of factors such as data quality, handling, and method performance.
Rationale for Selection
This protocol focuses on the development and implementation of machine-learning-based methods for kinase-specific prediction of protein phosphorylation sites. Machine-learning methods offer several advantages over traditional approaches:
- Ability to Capture Complex Signatures: Machine-learning algorithms can learn intricate patterns and interdependencies between residues, which are crucial for kinase-substrate recognition. This capability allows for the detection of subtle sequence features that may govern phosphorylation specificity.
- Utilization of In Vivo Data: Unlike consensus patterns and position-specific scoring matrices (PSSMs) that often rely on in vitro experiments, machine-learning methods can be trained on in vivo phosphorylation data. This enhances the relevance and accuracy of predictions by reflecting physiological conditions more closely.
- Adaptability to Diverse Data: Machine-learning models can accommodate diverse datasets and adapt to different biological contexts. They can integrate various types of information, including sequence motifs, structural characteristics, and protein-protein interactions, to improve prediction accuracy.
- Potential for Generalization: By learning from a diverse set of phosphorylation sites, machine-learning models can generalize well to predict kinase-specific phosphorylation events beyond the training dataset. This generalization capability is essential for robust and reliable prediction performance.
Material
Protein Sequence Dataset:
- The dataset consists of single or multiple protein sequences in FASTA format.
- Each sequence is preceded by a line starting with the ">" sign followed by the sequence name.
- Nonstandard amino acids, except the wildcard X, spaces, and line breaks, are ignored.
- Each sequence must be at least nine residues long, including the site of interest plus four residues on either side.
Prediction Services:
- NetPhos: http://www.cbs.dtu.dk/services/NetPhos/
- NetPhosK: http://www.cbs.dtu.dk/services/NetPhosK/
- NetPhosYeast: http://www.cbs.dtu.dk/services/NetPhosYeast/
Confidentiality:
- Query sequences submitted for prediction are kept confidential and will be deleted after processing.
- Alternatively, users can opt to keep their sequences in-house by utilizing stand-alone program packages for local use, which are available for most CBS programs.
Methods for Kinase-Specific Prediction of Protein Phosphorylation Sites
Selection of Prediction Method:
- Determine the origin of the query sequence data (mammalian, yeast, or other).
- For mammalian-origin data and generic phosphorylation prediction, utilize the NetPhos method.
- For kinase-specific predictions, use NetPhosK.
- If the query sequence data is of yeast origin, employ NetPhos-Yeast, specifically trained on yeast phosphorylation sites.
Input Sequence Submission:
- Paste the FASTA file directly into the sequence input field of the selected prediction server or upload it from the local drive.
- Ensure that each sequence in the dataset is properly formatted with a ">" sign preceding the sequence name.
Configuration of Prediction Settings:
- For NetPhos and NetPhosYeast:
- Choose the option to include graphical representation (default).
- Optionally specify the phosphoacceptor residues (S, T, or Y) for prediction (default: all).
- For NetPhosK:
- Customize prediction by defining a cutoff for the output scores (default: 0.5).
- Optionally activate the Evolutionary Stable Sites (ESS) filter to increase prediction reliability.
- Note that graphical output is not generated by default; enable the "Kinase Landscapes" option if graphical representation is desired.
Initiation of Prediction:
- Click the "Submit" button to initiate the prediction process.
- The job status (queued or running) will be displayed and continuously updated until completion.
- Once the prediction job terminates, the server output will appear in the browser window.
Interpretation of Prediction Results:
- NetPhos and NetPhosYeast:
- The output provides an overview of the input sequence, with predicted phosphorylation sites indicated in capital letters.
- Phosphorylation sites are listed with position, motif context, score (0.00–1.00), and final prediction.
- Graphical representation of the input sequence with color-coded bars indicating prediction scores for Ser, Thr, and Tyr residues is provided.
- NetPhosK:
- Prediction results are organized in columns indicating the predicted phosphorylation site and position, associated kinase, and score.
- A score above 0.5 typically indicates the likelihood of the site being targeted by a specific kinase, with higher scores suggesting stronger substrate matching.
- Interpretation of results may involve comparison with known kinase-substrate relationships and experimental validation.
Utilization of Additional Features:
- ESS Filter:
- Enhances prediction reliability by considering the presence of predicted sites in orthologous proteins of related species.
- Provides a list of potential phosphoacceptor sites along with their predicted kinases and scores.
- Kinase Landscapes:
- Presents predicted phosphorylation sites and associated kinases in a graphical format.
- Facilitates the identification of regions with an abundance or unusual composition of predicted phosphorylation sites and comparison of prediction outputs from related proteins.
Reference
- de Graauw, Marjo. Phospho-Proteomics. Humana Press, 2009.
