Nowadays, it is not overly expensive to get a synthetic construct for a gene of interest, with your codons optimized for E.Coli expression. I got a huge improvement in the expression of a protein - like 20-50-fold better than when I was trying to express with E.Coli Rosetta (which in principle help for the expression of rare codons). Take a look at GeneScript or GeneArt (LifeTechnologies), prices are quite competitive.

Good luck

Try using R2 cell..

What is your growth conditions and vector system?

Try growing in the low temperature (37C (OD600=0.8) for 3 hrs.. add iptg 0.1mM then grow @ 22C/80rpm O/N)

If its not solublized well in the buffer then try using Lysozyme before sonication. 

Some membrane or toxic proteins are difficult to get enough concentration.

let try to explain your conditions.. 

Few questions first:

Is it a soluble protein or a membrane protein? If you use a purification tag, which one is it and in which side of the protein? Is your protein full length or are you trying to express only a subpart of the protein?

And few clues:

Do you see a lot of your protein in the non soluble fraction in your SDS-Page? or no protein at all ? Synthetic gene or subcloning? Have you checked codon usage to avoid lots of "very rare" codons (sometimes usefull in an heterologus system)?

You can change the host system. IPTG is an inducer of BL21 DE3 strain. If possible, try GJ1158 strain. Its inducer is NaCl and media required for GJ 1158maintenance is saltfree media.

Actually...it very hard to express directly protein from eukaryotic protein into prokaryotic  expression system...it is because in prokaryotic system dont have translation modification step...so...better you extract the total RNA from that fungal...do RT-PCR to obtain cDNA sequence...before clone and express into e.coli..

Totally agree with @Abdul Rahim, I recommend to synthesize DNA sequence and optimize codon suitable for E-coli usage. Then, try different strains of the host to find the best one. Good luck.

You could also try auto-induction. You can make the media yourself, or just buy an auto-induction kit.

Nowadays, it is not overly expensive to get a synthetic construct for a gene of interest, with your codons optimized for E.Coli expression. I got a huge improvement in the expression of a protein - like 20-50-fold better than when I was trying to express with E.Coli Rosetta (which in principle help for the expression of rare codons). Take a look at GeneScript or GeneArt (LifeTechnologies), prices are quite competitive.

Good luck