小中大
You can consider other methods.
1. Check the gene information, which aa should be the site of phosphorelation? then you can use the specific antibody to this site (anti-aa-p, e.g. anti-tyrosine-p) to detect the phosporelation status in IP-WB.
2. If the protein have many phosporelation sites, you can try to run your IP protein on IF, the target protein position from WT and -/- may be in different pH area.